the rudimentary vertebrae of Centetes to the 40 or more well-developed
ones of Microgale.
The breast-bone, or sternum, is variable, but generally narrow, bilobate in front and divided into segments. The shoulder-girdle presents extreme adaptive modifications in the mole, in relation to the use of the fore-limbs in burrowing; but in the golden moles the fore-arm and fore-foot alone become specially modified. In Macroscelides the bones of the fore-arm are united at their lower ends, but in all other Insectivora the radius and ulna are distinct. The fore-foot has generally five digits; but in Rhynchocyon and in one species of Oryzorictes the first toe is absent, and in the moles it is extremely modified. The femur has, in most species, a prominent ridge below the greater trochanter presenting the characters of a third trochanter. In Tupaia, Centetes, Hemicentetes, Ericulus and Solenodon the tibia and fibula are distinct, but in most other genera united. The hind-foot consists usually of five digits (rarely four by reduction of the first), and in some, as in the leaping species (Macroscelides, Rhynchocyon), the tarsal bones are elongated. The form of the pelvis, and especially of the symphysis pubis, varies within certain limits, so that while in the Tupaiidae and Macroscelididae there is a long symphysis, in the Erinaceidae, Centetidae and Potamogalidae it is short, and in the Soricidae, Talpidae and Chrysochloridae there is none.
Owing to the similarity in the character of the food, the truly insectivorous species, forming more than nine-tenths of the order, present little variety in the structure of the digestive organs. The stomach is a simple, thin-walled sac; sometimes as in Centetes, with the pyloric and oesophageal openings close together; the intestinal canal has much the same calibre throughout, and varies from three (in the shrews) to twelve times (in the hedgehogs) the length of the head and body. In the arboreal Tupaia and the allied Macroscelididae, which probably feed on vegetable substances as well as insects most of the species possess a caecum. The liver is deeply divided into lobes, the right and left lateral being cut off by deep fissures; both the caudate and Spigelian lobes are generally well developed, and the gall-bladder, usually large and globular, is placed on the middle of the posterior surface of the right central lobe.
All the members of the order appear to be highly prolific, the number of young varying from two to eight in the hedgehog, and from twelve to twenty-one in the tenrec. The position of the milk-glands and the number of teats vary greatly. In Solenodon there is a single pair of post-inguinal teats, but in most species these organs range from the thorax to the abdomen, varying from two pairs in Gymnura to twelve in the tenrec. In the golden moles the thoracic and inguinal teats are lodged in deep cut-shaped depressions.
Scent-glands exist in many species. In most shrews they occur on the sides of the body at a short distance behind the axilla, and their exudation is probably protective, as few carnivorous animals will eat their dead bodies. In both species of Gymnura and in Potamogale large pouches are situated on each side of the rectum, and discharge their secretions by ducts, opening in the first-named genus in front of and in the latter within the margin of the vent. In the tenrec similarly situated glands discharge by pores opening at the bottom of deep pits.
The skin is thin, but in many species lined with well-developed muscles, which are probably more developed in hedgehogs than in any other mammals. In this family and in the tenrec most of the species are protected by spines implanted in the skin-muscle, or panniculus carnosus.
The Insectivora may be divided into two groups, according to the degree of development of the union between the two halves of the pelvis. The first group is characterized by the full development of this union, both pubis and ischium entering into the symphysis. The tympanum remains as Tree-Shrews. a ring within an auditory bulla; the orbit is either surrounded by bone, or separated from the hinder part of the skull by a post-orbital process of the frontal; the upper molars have broad 5-cusped crowns with a W-shaped pattern; and the intestine is generally furnished with a caecum. The first family of this group is the Tupaiidae, represented by the tree-shrews, or tupaias, of the Indo-Malay countries, characterized by the complete bony ring round the eye-socket, the freedom of the fibula from the tibia in the hind-limb, and the absence of any marked elongation of the tarsus. The dental formula is i. 23, c. 11, p. 33, m. 33, total 38. In appearance and habits tree-shrews are extremely like squirrels, although they differ, of course, in toto as regards their dentition. A large number of species are included as the typical genus Tupaia, which ranges from north-eastern India to the great Malay Islands. In these animals the tail has a fringe of long hairs on opposite sides throughout its length. In the pen-tailed, tree-shrew (Ptilocercus lowii), fig. 1, the only representative of its genus, and a native of Sumatra, Borneo and the Malay Peninsula, the fringes of long hair are confined to the terminal third of the tail. There are also differences in the skulls of the two genera. A third genus, Urogale, represented by U. cylindrura of the mountains of Mindanao, in the Philippines, and U. everetti, of Borneo, has been established for the round-tailed tupaias, in which the tail is uniformly short-haired, and the second upper incisor and the lower canines are unusually large, the third lower incisor being proportionately small, and also erect, while the second upper incisor resembles a canine. (See Tree-Shrew.)
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| Fig. 1. Pen-tailed Tree-Shrew (Ptilocercus lowii). |
In Africa the tupaias are apparently represented by the jumping-shrews, or elephant-shrews (so called from their elongated muzzles), constituting the family Macroscelididae. From the Tupaiidae the members of this family are readily distinguished by the fact that the socket of the eye, in place of having a complete bony ring, is separated from the hinder part of the skull merely by a post-orbital process of the frontal bone, and also by the more or less marked elongation of the tarsus or lower portion of the hind-limb; another feature being the union of the lower ends of the tibia and fibula. As indicated by one of their names, the members of the group leap after the fashion of gerbils, or jerboas, and hence walk much more on their toes than the majority of the order. In the typical genus Macroscelides, which ranges all over Africa and has numerous specific representatives, the dental formula is i. 33, c. 11, p. 44, m. 22 or 3, total 40 or 42; while there are five toes to each foot, and the lower ends of the radius and ulna are united. In Petrodromus (fig. 2) of East Africa, there are only four front-toes, and the hairs on the lower part of the tail form stiff bristles, with swollen tips; the dental formula being the same as that of those species of Macroscelides as have only two lower molars. A further reduction of the number of the digits takes place in the long-nosed jumping-shrews of the genus Rhynchocyon, which are larger animals with a much longer snout, only four toes to each foot, and a dental formula of i. 1 or 03, c. 11, p. 44, m. 22, total 36 or 34. Some of the species, all of which are East African, differ from the members of the typical genus by the deep rufous brown instead of olive-grey colour of their coat. (See Jumping-Shrew.)
In the second group, which includes all the other members of the order, the pelvic symphysis is either lacking or formed merely by the epiphyses of the pubes; the orbit and temporal region of the skull are confluent; and, except in the Talpidae and Chrysochloridae, the tympanum is ring-like, the tympanic cavity being formed by the alisphenoid and basisphenoid bones. The upper molars are triconodont, being either of the typical or a modified
