Brachiopoda. 



183 



the underside into a ganglion, from wtich the nerves proceed. There 

 are neither optic nor auditory organs. The vascular system is 

 well-developed ; a saccular heart lies above the digestive tract. The 

 excretory apparatus consists of one or two pairs of tubular 

 organs which open at one end into the body-cavity by a ciliated 

 funnel, and to the surface at the other exhibiting a great re- 

 semblance to the segmental organs of the Annelids. They 

 serve, at the same time, as a means of exit for the genital products, 

 which are formed on the wall of the body-cavity. The Brachiopoda 

 are of separate sexes. 



Pig. 144. 



Fig. 145. 



The ciliated larva swims 

 about freely. Its body is some- 

 times divided (Pig. 144) by 

 constrictions into segment- 

 like sections. Eyes may be 

 present at the front end, and 

 provisional bundles of bristles 

 (Pig. 145) behind. {Cf. the 

 Chsetopods) . 



The Brachiopoda are ex- 

 clusively marine; they are 

 as numerous in warm as in 

 cold seas ; there are, however, 

 but few species. They were 

 very numerous in early times, 

 and Are known from the Cambrian formations. They were well 

 represented in the Silurian, the Devonian, and the Jurassic. 



As examples may be cited : Terebratula, living as well as fossil, dorsal and 

 ventral sheUs convex, the former di-awn out into a beak-like process, pierced by 

 an apertm-e for the shoi-t peduncle, by wbicli tbe animal attacbes itself to 

 stones, etc. ; in other similar foi-ms there is a notch at the same place. Dorsal 

 valve with a loop-Uke brachial skeleton. Lingula, extant and fossil, two thin, flat, 

 homy, almost equal, hingeless shells ; peduncle very long, surrounded by a sandy 

 tube. 



Figs. 144 and 145. Larvae of two 

 Brachiopods. — After Lacaze - Duthiers and 

 Kowalevgky. 



