DEVELOPMENT OF THE EXTERNAL FORM — OPISTHOBRANCHIA. L65 



Veliger larva without shell or operculum, which presents a very 

 peculiar appearance (Fig. 73 A). This larva, in the length of its 

 body already shows a distinct approach towards the adult condition. 

 In the Tergipes oI.sci-v.mI by M. Schultze, the passage from the larva 

 to the adult is somewhat different, the velum degenerating in this 

 form before the shell is thrown off. lu this last case, the larva must 

 have adopted earlier the creeping manner of life. The shelldess larvae 

 of Tergipes Edwarddi, with their large vela at first swim about with 

 even greater rapidity than the shelled forms, but then gradually 

 begin to creep, as the body increases in size (Fig. 73 B). The lobes 

 of the velum commence to degenerate until they are reduced to a 

 pair of rounded processes lying in front of the mouth (Fig. 73 C), 

 which, it has been assumed, change into the labial palps. 



(L 



<J3. 



c. 



3. 



PlG. 7-!. — A-l>, Veliger larvae ami young of the Tergipes Edwardsii (after NORDMANN). 

 d, alimentary canal ; //. dorsal papillae. 



This view of the transformation of the remains of the velum into the sensory 

 organs near the mouth, has been adopted especially by Loven, who already 

 held a similar view as to the origin of the oral lobes in the Lamellibranchs 

 (p. 46). Ray Lankester holds that, in Limnaea, the remains of the velum 

 pass over into these subtentacular lobes ; but this point has been disputed 

 in connection with this form. It has already been stated (p. 133) that the 

 observations made by Ray Lankester for Onchidium were confirmed by 

 Joyeux-Laffuie. 



While the larva is still provided with the large velar lobes, one pair 

 of the dorsal appendages (cerata) arise which are so characteristic of 

 the Nudibranchs and into which the diverticula of the enteron soon 

 extend (Fig. 71 0). Another pair of these intestinal diverticula has 

 already formed and these belong to the next pair of cerata. As other 

 processes develop^ the young animal approaches more and more 



