24 BUSK, ON THE STRUCTURE AND 



closely envelopes the vitellus, is thin and firm ; the proper 

 vitelline membrane, whilst the outer is much thicker, and 

 according to him of a gelatinous consistence, swelling up 

 rapidly when the ova escape into the surrounding water. At 

 a later period it is sometimes absent, although the development 

 is not, according to Krohn, by this interfered with. Mr. 

 Darwin also (1. c.) assigns an outer envelope to the ovum, but 

 it would seem that this envelope was of a firmer consistence 

 than the one described by Krohn, since he states that it is rup- 

 tured soon after the commencement of partial segmentation of 

 the vitellus, which undergoes its further development after it 

 has thus escaped.* 



Many species of Sagitta are described by different authors, 

 but it would seem that considerable confusion still exists on 

 this subject. One thing appears tolerably certain, viz., that 

 the species common on the British coast, and which is the one 

 here desci"ibed, is, as before stated, very widely distributed in 

 all seas from the north to south antarctic oceans. And it may 

 well be supposed that superficial observation of specimens at 

 different ages and of different sizes, may have caused an unne- 

 cessary multiplication of species. 



Krohn, who considers that the number, position, and form 



* In Siebold and Kolliker's ' Zeitschrift f. Wissens. Zoologie,' Bd. v., 

 p. 15, is a short notice respecting the development of Sagitta, by C. 

 Gegenbaur. He states, that the process of segmentation terminates in 

 the production of an embryo of a rounded form, in which two liinds of 

 cell-masses may be recognized, — one central, constituted of minute, and a 

 well-defined peripheral layer, composed of larger cells. A depression is 

 now formed at one jDoint of the surface, which gradually advances to the 

 centre, constituting the rudiment of the intestine. The embryo now 

 appears to increase in length, in consequence of which, since it completely 

 fills the cavity of the ovum, it becomes bent, and is ultimately coiled in a 

 vermiform fashion. The cavity of the trunk may be distinguished, 

 traversed by the intestine, which forms, as it were, a vertical septum ; 

 but, besides tJiis, no other internal organs are a])parent. At this period 

 the embryo often moves, and on the addition of acetic acid the muscular 

 bands in the trunk are visible, completely formed, and exhibiting the fine 

 transverse strut. The fins arise as simple lateral outgrowths of the body. 

 In this condition the animal leaves the ovinn, about f" in length, and 

 already presenting in all resjiects the character of the full-grown Sagitta. 

 The other organs, consequently, are not developed until after the animal 

 has quitted the ovum. In the entire course of development, many stages 

 of which, particularly those which succeed complete segmentation, are 

 very difficult to be understood, cilia never make their ajiiiearance. 



If the anatomy of this creature had not satisfactorily shown that it 

 belongs neitlier to the Pteropoda, nor to the Heteropoda — this would have 

 been rendered certain by its mode of development, which does not accord 

 in any respect with the MoUuscan type. What the real position of Sagitta 

 is I will not determine. 



