THE AMPHINEURA 59 



gastric commissure is given off from the cerebral mass, and enters 

 a pair of small ganglia situated in the median line between the 

 oesophagus and the radular sac ; moreover, the two same upper 

 buccal and subradular commissures as in the Chitones are present, 

 at least in Pronrmnenia and lihopalomenia (Heath). 



The only organs of special sense are the club-shaped epithelial 

 papillae in the thickened cuticule of the Proneomeniidae, Neomenia, 

 and the following Parameniidae, Dinomenia, Proparamenia, and 

 Paramcnia. There is also a median invaginable sensory papilla 

 situated on the dorsal posterior surface, above the rectum. It is 

 not covered by the cuticle, and may be multiple, as in Dondersia, 

 or absent, as in Hemiinenia and perhaps Strophomenia. 



Generative Organs. The Neomeniomorpha are hermaphrodite. 

 The gonads are paired tubular structures, separately opening 

 posteriorly into the pericardium (Fig. 30, C) and extending forward 

 close under the dorsal sinus to near the front end of the body. The 

 ova are developed on the median, the spermatozoa on the outer 

 wall of each gonad. The sexual products pass into the pericardium, 

 whence they are driven by the pericardial cilia into the kidneys, 

 which, as has been explained above, serve as gonaducts and are 

 provided with receptacula seminis and shell -glands. In several 

 species paired excitatory organs, in the form of protrusible cal- 

 careous spicules, are situated in the cloacal chamber on either 

 side of the uro-senital orifice. 



III. EMBRYOLOGY. 



Little is known of the development of the group. The ova are 

 laid separately. In My.omenla banynlensis (Pruvot) 

 the segmentation is regular, and an invaginate 

 gastrula Avith an originally posterior blastopore is 

 formed. An anterior ciliated ring or velum is 

 formed, having in its middle a tuft of cilia, which 

 is eventually transformed into a single terminal 

 tiugellum (Fig. 17). The external surface of the 

 trochophore larva thus formed is made up of a 

 number of ciliated test-cells, which as in the 

 Lamellibranchiate Yoldia eventually are cast off 



KIG. 30. 



and the mantle is formed beneath. The post- 



... | . An advanced larva 



velar region ot the embryo elongates, and its of Myzomenia imnyu- 



ectoderm cells develop spicules. Finally, the "after 6 thTTes't 



velum disappears, and seven imbricated calcar- p^"" ) 11 off< (After 

 eous plates, made up of more or less flattened, 



closely - apposed spicules, are formed on the dorsal surface 

 (Fig. 39). 



