260 



NATURAL HISTORY. 



and more highly organised division of Tretenterata,* are characterised by the presence of 

 an anal orifice. Those belonging to the second and lower group of Clistenteratat are not 

 provided with that opening, and the intestinal tube ends in a blind sac. These important dis- 

 tinctive characters, first made known by the anatomical researches of Messrs. Hancock and 

 Huxley, have since been confirmed by Professor Morse in a living Clistenterate. He 

 witnessed the rejection of the waste products by the mouth, the only outlet in one of the 

 lower forms (Terebratulina). Other remarkable structural differences are associated with these 

 distinguishing features. The first, or Tretenterate group, is devoid of any internal skeleton, the 

 two valves of the shell being quite free, and kept in place solely by the shell muscles. In at least 

 one genus of it (Lingula), they move freely from side to side, opening obliquely by means of the strong 

 " lateral " or side muscles, specially restricted to that purpose (Fig. 6, j k I). Members of the 

 second group generally have the two valves firmly united by hinge teeth, which fit into sockets 

 in the opposite valve and effectually prevent any lateral movements. Consequently, the side 

 muscles are not developed, but are replaced by others which enable the animal to open its shell 

 a very little way in a horizontal direction; or, in one genus (Thecidium) at right angles. But 

 the gape is exceedingly small amongst the Brachiopoda, -as compared with that of the true 

 bivalved mollusca. An internal skeleton, for the support of the breathing organs, is usually present 

 in the Clistenterata, and the shape it assumes is most variable. As the Tretenterates appeared 

 first on the globe, the second and inferior forms may be regarded as possibly their degenerated 

 descendants. Members of one ancient and extinct family, | believed to be exclusively restricted 

 to the seas of the Silurian age, present a general external resemblance to the lower and 

 Clistenterate species. They appear also to have possessed some characters of both groups, 

 rudimentary hinge teeth, indicating interlocking valves, being associated with the impressions 

 left by side muscles which may have enabled the animal to move its shell sideways. The 

 chemical constituents of these often massive shells are also of a different and somewhat intermediate 

 nature, carbonate of lime entering far more largely into their structure than into that of Lingula and 

 Discina, which are almost exclusively composed of a corneous or horny substance ; while in the shells 

 of Crania, Glottidia, and all the Clistenterate genera the calcareous element predominates. 



The shell is secreted by the mantle, and appears (in Terebratulina) in the fifth stage 

 of the development of the hitherto naked embryo. It first develops in the region of the peduncle, 

 subsequently increasing in growth at the margins of the valves, and it differs so considerably, 

 both in structure and mode of growth, from that of the lamellibranch, that a small fragment 

 can be readily identified by microscopic examination, even when derived from a fossil species. 

 Dr. Carpenter describes the shell as consisting of two layers which correspond in thickness with 

 the outer layer only of that of the lamellibranch. It has a fibrous prismatic structure 

 which gives it a scale-like appearance (Fig. 4, A). In many genera the shell is perforated 

 with minute canals, differing in size and situated at variable distances from each other. 



They make their appearance in the earliest stages 

 of the development of the shell ; the largest 

 measure -^1^ and the smallest g-oVo of an inch 

 in diameter. These canals are charged with cellu- 

 lar projections (Fig. 4, B, c) of the fleshy mantle, 

 corresponding in position with tiny cells spread 

 over the upper surface of this delicate mem- 

 branous covering of the animal. Their functions 

 are not absolutely determined, but Dr. Carpenter 

 believes them to be subservient to respiration. 



Of the two valves composing the shell, the 

 front or ventral valve (Fig. 5, A) is generally the larger. In many forms it is produced at the apex (a) 

 into a prominent beak perforated at its extremity by a hole, the foramen (/), for the passage of the 

 peduncle or bundle of muscular fibres by means of which the animal attaches itself to neighbouring 

 objects. The back shield, or dorsal valve (Fig. 5, B), contains the animal, and also a variously-shaped 

 * Greek, tretos, perforated. f Greek, deistos, shut ; entera, intestines. J The Trimerellidae. 



A x C B 



Fig. 4. SECTIONS OF SHELL STRUCTURE. C, C-ffiCAL TUBULI 



OR PROJECTIONS. (After Carpenter.) 



