DEVELOPMENT OF THE EXTERNAL BODY OPISTHOBRANCHIA. 163 



withdrawn into it ; the retention of the operculum as in Adaeon 

 (Tornatella} is quite exceptional. According to TRINCHESE, the 

 larval shell in some forms (Saccoglossa) shows a delicate reticulate 

 structure on its surface ; in most other larvae it is smooth. 



Passing now to the internal organisation of the Veliger larva, we 

 notice first that, from the oral aperture which lies at the ventral 

 incision of the velum, the stomodaeuni (which only at a later stage 

 is provided with a radula) runs backward and becomes connected 

 with the large enteron. From this latter, there are two lateral 

 outgrowths which differ in size (Fig. 72, di) ; these are formed of 

 specially yolk-laden cells and thus no doubt owe their origin to 

 the macromeres. The intestine also arises as a diverticulum of the 

 entoderm-sac ; it then lengthens considerably, bends forward and, 

 after uniting with the ectoderm at the right side of the body, opens 

 outward rather far forward, near the edge of the shell (Fig. 72). 



Little is as yet known as to the differentiation of the mesoderm in 

 the Opisthobranchia. A strong muscle, sometimes composed of two 

 branches, runs back from the velum, becoming attached to the shell 

 posteriorly (Fig. 72, mu}. Another shorter retractor of the velum 

 extends between the base of this organ and that of the foot. This 

 arose from single spindle- or star-shaped mesoderm -cells which came 

 to lie on the right side of the larva in this region. 



This latter muscle carries on regular rhythmical movements and, on this 

 account, has, according to TRINCHESE, falsely been regarded by several 

 observers as a heart. The so-called larval heart which has been described 

 in connection with the Prosobranchia (Nassa, Fig. 61 E, Ih, p. 150) differs 

 somewhat in position from this retractor, but is, like it, composed of long 

 mesoderm-cells. 



According to the accounts of the Opisthobranchs now under con- 

 sideration, no primitive kidneys resembling in shape those occurring 

 in the Prosobranchia (Paludina) and Pulmonata (p. 136) have been 

 found in them, but vesicular structures which lie in the dorsal 

 region behind the velum to the right and left of the oesophagus 

 have been described as primitive kidneys. These have been regarded 

 as excretory organs chiefly because they are filled with strongly 

 refractive concretions. They seem never to possess efferent ducts.* 



The views taken of the excretory organs of thre Opisthobranchia seem 

 to us to be somewhat confused. TRINCHESE, for instance, has described a 

 paired or unpaired sac-like gland with a longer or shorter efferent duct which 



* [These appear to be ectodermal in origin (HEYMONS) and analogous to the 

 ectodermal anal kidney of the Prosobranchia (p. 129 and No. XV.). Ed.] 



