184 



COMPARATIVE ANATOMY 



CHAP. 



which is prolonged backwards like a stalk. Body cavity little developed. Colonial, 

 in tubes which raise themselves on a common creeping stem. Rhabdopleura, 

 Cephalodiscus is a related form. 



Sub-Order 2. Ectoprocta. 



Anal opening outside of the tentacle carrier. Tentacle carrier not produced. 

 Anterior body naked, posterior body shelled. Without stalk. Anterior body so 

 enveloped in a fold of the posterior body as to be either temporarily or permanently 

 surrounded by a sheath (tentacle sheath) out of which it can be protruded. Body 

 cavity tolerably spacious. Shell often calcareous. Colonial. Phyladolcemata. 



Tentacle carrier horseshoe-shaped. In- 

 habit fresh water. Cristatella, Alcy- 

 onella, Fredericella, Lophopus (Fig. 

 122), Plumatella (Fig. 139, p. 208). 

 Gymnolcemata. Tentacle-carrier circu- 

 lar. Live, with the exception of Palu- 

 dicella, in the sea. Cellepora, Eschara, 

 Bugula, Fhistra, Alcyonidium, Hor- 

 nera, etc. 



Sub-Order 3. Entoprocta. 



Anal opening inside the tentacle 

 carrier. A tentacle sheath wanting. 

 Body stalked. With one pair of neph- 

 ridia. Body cavity reduced. Pedi- 

 cellina, colonial. Loxosoma, living 

 singly. Marine. 



Fio. 122. Anterior body of Lophopus (after 

 Allman), from the right side, t, Tentacles cut off 

 near the base ; o, mouth ; ep, epistome ; st, fore- 



gut ; ga, ganglion ; an, anus ; pr, hind-gut, 

 free ends of the tentacle-carrier are cut off. 



The 



Order 4. Brachiopoda (Fig. 178, p. 269). 



The dorsal and the ventral body 

 walls form a fold directed to the front, 

 so that the body is covered by dorsal 

 and ventral mantle folds, which may 

 coalesce behind and at the sides. The mantle folds secrete calcareous, and occasionally 

 horny shell valves, a dorsal and a ventral. The ventral valve is generally the more 

 concave. At the sides of the mouth are inserted the two long oral arms set with 

 lateral cirri ; these arms lie spirally rolled up in the mantle cavity, which is 

 formed by the mantle folds, and are often supported by a special calcareous skeleton 

 united to the dorsal valve. Anus wanting, or lies asymmetrically and anteriorly, 

 to the right, near the mouth. In Crania only it lies quite behind, in the dorsal 

 middle line. Central nervous system is an oesophageal ring with weakly developed 

 brain and infra-cesophageal ganglionic swellings. One pair (less frequently two pairs) 

 of nephridia, at the same time ducts for the transmission of the sexual products, open 

 to the right and left of the mouth into the mantle cavity. Blood-vascular system 

 probably present, with a heart placed above the intestine. The posterior end of the 

 body is often prolonged into an attached stalk, which emerges either between the 

 shell valves (Lingula), or through a hole in a posterior upward bulging of the larger 

 ventral valve. In many cases the stalk is wanting, and the shell is fastened to the 

 surface on which it rests direct by the ventral valve. Exclusively inhabitants of the 

 sea. The larger proportion of the species and genera are fossil. The 

 has maintained itself since the Paleozoic epoch. 





