Brooks.] 230 [February 2, 



pendent origin of similar structures; a class of relations which has 

 not yet been sufficiently allowed for in the speculations of the modern 

 school of zoology, but which seems destined to form, at some future 

 time, an important element in the theory of the evolution of life. 

 The superiority of the conceptions of Allman becomes evident as 

 soon as we contrast them with many which have been advanced; for 

 example, the comparison advocated by a very distinguished natura- 

 list and embryologist between the foot of a Lamallibranch, the tail of 

 Appendicularia, and the placenta of Salpa. 



We come now to the question : if our present knowledge of the 

 embryology of the Mollusca and Molluscoida disproves all the old 

 ideas of their affinity, does it present any thing to replace them? 



Most of the Gasteropoda are known to pass through a free, locomo- 

 tive " Veliger " stage. The veligers of different Gasteropods differ 

 considerably in form ; and in some the embryo, at this stage, is much 

 less specialized than in others; but, omitting the complications intro- 

 duced as adaptations to a spiral shell, the veliger of such a marine 

 Gasteropod as Astyris may be regarded as presenting the typical 

 form. A veliger may be described as a free-swimming, bilaterally 

 symmetrical embryo, without a true heart or vascular system, or 

 branchiae, with the mouth and anus near each other on the median 

 line. The digestive organs are suspended in the body cavity, and 

 attached to the body-wall at the two external apertures, and by the 

 various muscles. The foot is situated between these two openings; 

 and the pedal ganglia, which are in most veligers the first ganglia to 

 appear, are developed in the region of the foot; that is, between the 

 mouth and the anus. The foot is generally supplied with a bunch of 

 setae, which are apparently sensory in function. The animal is 

 inclosed in a shell composed of two portions ; a large ventral cup, 

 and a neural or pedal operculum, which is united to the anal margin 

 of the cup at the earliest stages, and subsequently becomes separated 

 from it. This shell and lid are found in the embryos of those forms 

 where the adult is without an operculum, as Crepidula, as well as in 

 those where the adult is destitute of a shell, as the Nudibranchs. 



The most characteristic peculiarity of the veliger is the velum. 

 This is a large, bilaterally symmetrical circlet of cilia, developed 

 from the cephalic region of the embryo, and supported, at some dis- 

 tance from the body, by a transparent double-walled veil, the cavity 

 of which is irregularly divided into large sinuses, in free communica- 

 tion with the body-cavity. The animal swims, usually near the sur- 



