6l8 THE INVERTEBRATA 



and fixed by its whole surface to a rock; the dorsal valve is conical. 

 The tentacles of the lophophore are protruded from the shell margin. 



The Brachiopoda have free-swimming larvae which are usually 

 divided into three regions, an anterior like the preoral region of the 

 trochosphere, a median region in which the two lobes of the mantle 

 are early produced, and a posterior one, hidden by the mantle lobe, 

 which becomes the stalk (Fig. 427 B). The mantle lobes develop four 

 bundles of chaetae (Fig. 427 C), and then turn forward to envelop 

 the anterior region (Fig. 427 D). This now begins to develop the 

 lophophore (Fig. 427 E, F, G) and shell valves form on the mantle 

 lobes, while the posterior region grows into the stalk. 



The coelom develops as a pair of pouches or a single pouch from 

 the archenteron (Fig. 427 A). Though the presence of mantle lobes, 

 the presence of chaetae and the resemblance of the larva to a trocho- 

 sphere relates the Brachiopoda to the annelid-mollusc stock, there 

 is no evidence of segmentation and they cannot come very close to the 

 Annelida ; but possibly are nearer to the MoUusca. On the other hand 

 the enterocoelic development of the body cavity suggests affinities 

 to the echinoderms and chordates. 



Classification 



EcARDiNES. Brachiopoda having shells with no hinge, no internal 

 skeleton, and alimentary canal with an anus. Lingula^ Crania. 



Testicardines. Brachiopoda having shells with hinge and internal 

 skeleton, without anus. Terehratula^ Waldheimia. 



PHYLUM CHAETOGNATHA 



Coelomate animals with an elongated body divided into three regions, 

 head, trunk and tail, and with lateral and caudal fins ; head with a 

 pair of eyes and two groups of chitinous teeth and jaws; cerebral 

 ganglion and ventral ganglion (in the trunk) connected by circum- 

 oesophageal commissures ; body wall containing a layer of longitudinal 

 muscle cells of peculiar type arranged in four quadrants ; alimentary 

 canal straight ; no localized excretory or respiratory organs or vascular 

 system; hermaphrodite and cross-fertilizing; free-swimming larva. 



The structure of an individual of this small and homogeneous 

 group is shown in Fig. 428. Very little need be added to the definition. 

 The muscles are of a primitive type, each elongated cell consisting 

 of a core of unmodified cytoplasm and an outer shell ring of con- 

 tractile substance ; they have thus some resemblance to those of the 

 nematodes. The chaetognaths are, however, capable of executing 

 very rapid movement by suddenly contracting these longitudinal 

 muscles and are able to pounce upon and capture their food, which 



