560 INVERTEBRATA CHAP. 



and extinct. Thus the stalk is retained throughout life in the living 

 genera Pentacrinus, Rliizocrinus, Batliycrinus, and Hyocrinus, and 

 in the vast majority of fossil forms. The basals remain exposed in 

 Thaumatocrinus, Ehizocrinus, Batliycrinus, Hyocrinus, and in an 

 enormous number of fossil forms. Lastly, in many fossil forms the 

 under-basals are distinct and exposed, and the nerves running from 

 the aboral nervous system, instead of being enclosed in the ossicles, 

 run in open grooves in them. 



ANCESTRAL SIGNIFICANCE OF LARVAE OF ECHINODERMATA 



We may now pass in review the larvae of Echinodermata viewed 

 as a whole. We may discount the characteristic features in the 

 development of Antedon, which are obviously due to the yoiky egg 

 and incapacity of the free-swimming larva to take food, and regard 

 it as tolerably certain that, i we were lucky enough to find a (Jrinoid 

 with a small yolkless egg, that this would develop into a larva 

 fundamentally similar to the larvae of Asteroidea, Echinoidea, 

 Ophiuroidea, and Holothuroidea. These larvae agree in possessing 

 as a locomotor organ a folded longitudinal ciliated band with prae-oral 

 and anal loops ; a V-shaped adoral ciliated band which apparently 

 removes surplus food from the region of the mouth ; an alimentary 

 canal consisting of shovel -shaped buccal cavity (stomodaeum), 

 oesophagus, globular stomach, and intestine ; and a coelom, which 

 originates as a pouch from the anterior end of the gut, which divides 

 into right and left halves, and which communicates with the exterior 

 by a ciliated canal opening on the dorsal surface to the left side of 

 the middle line. 



Now, when we find a group of larvae exhibiting so many characters 

 in common, we conclude that they represent a common ancestor 

 from which the groups to which they belong are descended. To this 

 hypothetical ancestor we may give the name Dipleurula. Since the 

 larvae in question, so far as their external features go, are perfectly 

 bilaterally symmetrical, we may regard those of them whose internal 

 organs exhibit the nearest approach to bilateral symmetry as, in this 

 respect, retaining the characters of the ancestor. It follows that the 

 larvae of Ophiuroidea are in this respect most primitive, for it is in 

 every way likely that the internal asymmetry displayed so markedly 

 by the Auricularia larva, and in much lesser degree by the Bipinnaria 

 and the Echinopluteus, is an anticipation of the adult arrangement, 

 where the organs of the left side overpower those of the right. 



We may credit then the ancestral Dipleurula with a coelom 

 which, on each side, was perfectly or imperfectly divided into three 

 divisions : an anterior, a middle, and a posterior. The middle 

 division was prolonged into lobed processes, covered externally with 

 ciliated epithelium, which projected at each side of the mouth and 

 produced currents which transported small pelagic organisms, on 

 which the animal fed, to the mouth. In a word, the two hydrocoeles 



