590 



INVERTEBRATE MORPHOLOGY. 



differences are associated with the absence or reduction of the ovoid gland 

 and of an aboral nervous system. The number of the organs is very much 

 reduced, and no genital cords have as yet been discovered. It is interest- 

 ing to note, however, the existence of a genital lacuna mentioned above, 

 in association with which the reproductive organs seemto develop, and it 

 may be, as stated, that the lacunar thickening from which it arises is to be 

 regarded as representing the ovoid gland, which, as has been seen, is 

 intimately connected with the lacunar system in other forms. It seems 

 probable that in harmony with the shortening of the stone-canal and its 

 separation from the body-wall, and with the abortion of the axial entero- 

 coel, there has been a shortening of the genital cords so that the aboral 

 ring no longer exists, and the reproductive organs, reduced in number, 

 develop directly upon the wall of the geuital lacuna. It must be remarked 

 that in some forms there is no distinct genital lacuna, but the reproductive 

 organs are associated with the intestinal lacunae, a condition which may 

 be secondary. 



Development of the Solothuroidea. — The typical larva of the Holothu- 

 rians is known as the Auricularia (Fig. 270), and is distinguished from 



that of the Asteroids, Ophiuroids, and 

 Echinoids by being destitute of armlike 

 processes. In later stages the ciliated 

 bands fuse in such a manner as to form 

 a series of circular bands surrounding 

 the barrel-shaped larva and recalling the 

 larva of the Orinoids. By the gradual 

 elongation of this larva and the disap- 

 pearance of the ciliated bands the adult 

 form is acquired, there being no absorp- 

 tion of any extensive portion of the larval 

 body as in the Brachiolaria and Pluteus. 

 The Phylogeny of the Echinoderma. 

 — The Echinoderms form a well-defined 

 group showing little indication of alfini- 

 PiGf. 370 —AuRicuLAEiA Larva OF ties with other forms, and the establish- 



Synapta (after Semon). 

 dp = dorsal pore. 

 H = hydroccel. 

 pi = primary tentacles. 

 St = secondary tentacles. 



ment of a plausible phylogeny is an 

 unusually difficult task. One thing, 

 however, seems certain from their de- 

 velopmental history, and that is that they 

 have been derived from primitive bilat- 

 eral forms, and that the radiality charac- 

 teristic of the adults has been secondarily acquired. The larvae are strictly 

 bilateral, there being indications that originally two water-pores, situated 

 symmetrically upon the dorsal surface, existed. The first question to be 

 decided then is the cause of the radial symmetry seen in the adult. 



Bilaterality in the animal kingdom is usually associated with an antero- 

 posterior differentiation, and this with a definite axis of progression. Thus 



