Chap. IV. DIFFERENT TYPES OF LARVAE. 59 



and Asteracanthion pallidus — species which have their representatives in Europe, and 

 which have, up to the present time, been included in the same genus with Astera- 

 canthion Mulleri — are free-swimming larvae, resembling the Bipinnaria of Miiller. 

 These facts can, therefore, leave but little doubt that Miiller and Van Beneden have 

 observed the larvae of Asteracanthion rubens M. T., and of allied species ; the larvae 

 of which have been called by them Bipinnaria, Brachiolaria, and Brachina, and are 

 only different stages of one and the same generic type. The difference of the two 

 modes of development of A. Mulleri and A. pallidus is so great, that these two groups 

 of species have been separated into two genera by Professor Agassiz. The Brachio- 

 laria from Triest and Messina present very striking differences, from the northern 

 Brachiolaria. These larva? are probably the young of Asterias tenuispinus, which is 

 so common in the Mediterranean. In his revision of the Starfishes, Professor Agassiz 

 has also separated this species from the true Asteracanthion, under another generic 

 name. We have next the Bipinnaria asterigera, still another type of larva, belonging 

 in all probability to another family, differing from both the other larval forms. As 

 Bipinnaria asterigera can only be the larva of a Pteraster, Ctenodiscus, an Astropecten, 

 or of an Hippasteria, either of which belong to families distinct from the Brachiolaria 

 type of larvae, we find differences in form, modified by structural features, character- 

 izing the larval conditions, as well as the adult stages of families of the same 

 order ; while structural peculiarities in the larvae, characterize the different generic 

 divisions more plainly than in the more advanced conditions. It is evident, from 

 the observations of Professor Agassiz and of Sars, that the Asterias violaceus of 

 Thompson, of which he has traced the embryology in the Microscopical Journal, must 

 be placed in the same genus with A. Mulleri, and may, perhaps, be identical with it; 

 unless the true A. violaceus L. has also a similar mode of development. There is still 

 another type of Echinoderm larva?, which in all probability are the larva? of Star- 

 fishes, — viz. : the Tornaria type. In this type there is not the excessive development 

 of the ciliary chord into long, slender arms, characteristic of the Brachiolaria; there 

 are only slight, wavy indentations, corresponding to the position of the arms of the 

 Brachiolaria, as we find them in the younger stages of the larva? (PL III. Fig. 4 ; 

 PI. II. Fig. 26). In fact, this type of larva, in its adult condition, seems to be a 

 permanent embryonic type of the younger stages of the Brachiolaria. I would infer 

 from this that the Tornaria will probably prove to be the larva of Ctenodiscus, 

 Astropecten, or Saya, or of some Starfish with pointed ambulacral suckers. Having 

 had the opportunity to examine several of the Tornaria type of larvae at Naushon, 

 in different stages of development, I hope to return to this subject at a future 

 time. 



