480 COMPARISON OF ECHINODERM LARV.E. 



Originally, no doubt, the transition from the larva to the adult 

 was very simple, as it is at present in most Holothuriaus ; but as the 

 larvae developed various provisional appendages, it became necessary 

 that these should be absorbed in the passage to the adult state. 



It would obviously be advantageous that their absorption should 

 be as rapid as possible, since the larva in a state of transition to the 

 adult would be in a very disadvantageous position. The rapid meta- 

 morphosis, 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. 478), 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 

 Chsetopoda, Mollusca, etc., but the differences between the two types 

 are more striking than the resemblances. It firstly differs from the 

 trochosphere larva in the chai^acter of the ciliation. Both larvae start 

 from the iiuiformly ciliated condition, but while the prae-oral ring is 

 almost invariable, and a peri-anal ring very common in the trocho- 

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

 even when present, i.e. the prre-oral ring of Bipinnaria and the ter- 

 minal 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 Echinoderm 

 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 larva? are of no importance in the 

 comparison between the trochosphere larvae and the larvae of Echino- 

 dermata, since such rings are frequently secondarily developed. Cf. 

 Pneumodermon and Dentalium amongst Mollusca. 



In the character of the pree-oral lobe the two types ao-ain differ. 

 Though a prae-oral lobe is often found in Echinoderm larvas it is never 

 the seat of an important (supra-cesophageal) ganglion and organs of 

 special sense, as it invariably is in the trochosphere. 



Nothing like the vaso-peritoneal vesicles of the Echinoderm larvse 

 has been found in the trochosphere ; nor have the characteristic tro- 

 chosphere excretory organs been found in the Echinoderm larvae. 



The larva which most nearly approaches those of the Echino- 

 dermata is the larva of Balanoglossus described in the next chanter. 



BIBLIOGRAPHY. 



(542) Alex. Agassiz. Revision of the Echini. Cambridge, U.S. 1872 74. 



(543) Alex. Agassiz. "North American Starfishes." Memoirs of the Museum of 

 Comparative Anatomy and Zoology at Harvard College, Vol. v., No. 1. 1877 (oriinally 

 published in 1864). 



(544) J. Barrois. "Einbryogenie de 1'Asteriscus verruculatus. " Journal de I'Anat 

 et Phyx. 1879. 



