576 COMPARISON OF ECHINODERM 



position. The rapid metamorphosis, which we find in Asteroids, 

 Ophiuroids, and Echinoids in the passage from the larval to the 

 adult state, has no doubt arisen for this reason. 



In spite of the varying provisional appendages possessed by 

 Echinoderm larvae it is possible, as stated above (p. 574), to 

 recognise a type of larva, of which all the existing Echinoderm 

 larval forms are modifications. This type does not appear to 

 me to be closely related to that of the larvae of any group 

 described in the preceding pages. It has no doubt certain 

 resemblances to the trochosphere larva of Chaetopoda, Mollusca, 

 etc., but the differences between the two types are more striking 

 than the resemblances. It firstly differs from the trochosphere 

 larva in the character of the ciliation. Both larvae start from the 

 uniformly ciliated condition, but while the prae-oral ring is almost 

 invariable, and a peri-anal ring very common in the trochosphere; 

 in the Echinoderm larva such rings are rarely found ; and even 

 when present, i.e. the prae-oral ring of Bipinnaria and the terminal 

 though hardly peri-anal patch of Antedon, do not resemble 

 closely the more or less similar structures of the trochosphere. 

 The two ciliated ridges (fig. 264 A) common to all the Echino- 

 derm larvae, and subsequently continued into a longitudinal ring, 

 have not yet been found in any trochosphere. The transverse 

 ciliated rings of the Holothurian and Crinoid larvae are of no 

 importance in the comparison between the trochosphere larvae 

 and the larvae of Echinodermata, since such rings are frequently 

 secondarily developed. Cf. Pneumodermon and Dentalium a- 

 mongst Mollusca. 



In the character of the prae-oral lobe the two types again 

 differ. Though the prae-oral lobe is often found in Echinoderm 

 larvae it is never the seat of an important (supra-oesophageal) 

 ganglion and organs of special sense, as it invariably is in the 

 trochosphere. 



Nothing like the vaso-peritoneal vesicles of the Echinoderm 

 larvae has been found in the trochosphere ; nor have the charac- 

 teristic trochosphere excretory organs been found in the Echino- 

 derm larvae. 



The larva which most nearly approaches those of the Echino- 

 dermata is the larva of Balanoglossus described in the next 

 chapter. 



