698 



ZOOLOGY 



SECT. 



sive cavity, lined with a coelomic epithelium, and divided into three 

 completely separated parts the pericardium, the genital cavity, 

 and the general body-cavity. 



Vascular System. The vascular system of the Aplacophora 

 is very rudimentary. There is a heart enclosed in a pericardium 

 (Figs. 612 and 615, peri) and composed, when best developed, of 

 an auricle and a ventricle. 



In Chiton there is a well-developed heart (Fig. 613, Jit.) consisting 

 of a median ventricle and two lateral auricles. The pericardial 

 cavity in which it lies is a space of considerable extent in the 

 posterior region of the body, below the two last valves of the shell. 

 The Nervous System consists in the Aplacophora (Fig. 614, 

 A,B,C) of four longitudinal nerve-cords two pedal and two pleural. 

 These are connected together by an cesophageal ring, thickened 

 dorsally into a single or double cerebral ganglion ; and in front of 

 this is a second, more slender stomatogastric nerve-ring with small 



ganglia. The 

 pedal cords 

 (v.v) may pre- 

 sent in front a 

 pair of gang- 

 lionic thicken- 

 ings connected 

 by a commis- 

 sur e, and 

 further back 

 there may be a 

 series of e n - 



FIG. 614. Nervous system of Amphineura. A, Proneomenia ; large ments 



B, Neomenia; C, Chcetoderma ; I), Chiton, c, cerebral ganglia; + j u. r nnr>1 _ 



/, I, pleural cords ; pc. posterior commissure ; s, stomatogastric "j ( 



commissure or ring, with ganglia; v, v, pedal cords. (From the mieciirp's Tlip 

 Cambridge Natural History, after Hubrecht.) 



pleural cords 



(I, I) are connected behind, above the rectum, by a commissure 

 (p, c) which usually bears a median enlargement. Sometimes a 

 union takes place posteriorly between the cords of the two pairs. 

 There are no eyes, or statocysts, or tentacles. The dorsal epidermal 

 papillae are perhaps sensory. Some have a sensory frontal lobe and 

 a sensory pit or elevation in the middle line of the dorsal surface 

 near the posterior end. 



In the Placophora (Fig. 614, D) there is an cesophageal nerve- 

 ring consisting of a thicker dorsal cerebral portion not differentiated 

 into ganglia, and a thinner ventral buccal commissure. The cerebral 

 part sends off nerves to the labial palps, the lips, and the buccal 

 apparatus. Two pairs of longitudinal nerve-cords, pedal and 

 pleural, are given off posteriorly : the former, from which arise 

 nerves to the foot, are joined by numerous commissures passing 

 beneath the enteric canal ; the latter, which send off nerves chiefly 



