Parasitic Crustacea effect their Conservation. 275 



the Chondracanthi, on the contrary, are packed together in 

 layers. The eggs never contain more than one vitellus. 



The young Crustacea of these various species do not disperse 

 themselves immediately after their escape from the egg ; they 

 remain for some time fixed upon the oviierous tubes, from which 

 they afterwards dart in pursuit of tlieir prey, or establish them- 

 selves upon the surface of the fish on which they have been 

 hatched. 



They swim rapidly and in gyrations, by means of the six bi- 

 ramose feet terminated with long hairs, which they agitate with 

 great force. I have remarked that when an opake body is 

 passed rapidly above the vessels in which they are kept, their 

 movements become much more rapid, which leads me to think 

 that the shadow resulting from the interposition of this body 

 between them and the light produced for them the same eflPect 

 as that of the passage of a fish within their reach, and which 

 they endeavoured to seize. 



In the centre of these young embryos we observe the stomach, 

 which, not being yet filled with food, appears nevertheless to be 

 distended, as if it contained air, and may assist in facilitating 

 progression by sustaining them and performing the office of a 

 swimming-bladder. 



The young Crustacea, after their escape from the t^^, may 

 exist without nourishment for from three to fifteen days when 

 they are preserved in vessels filled with very pure sea- water and 

 kept in a dark and cool place. There are species which live for 

 a considerable time ; but generally the embryos of the Pandoree 

 and Chondracanthi die before those of the Trebia and Caligi, 

 which, moreover, are more lively in their movements than those 

 of the former Crustaceans. I have also ascertained that the life 

 of the embryos attached to their mothers by a frontal cord lasted 

 much longer than that of those preserved separately in water, 

 which they survived for a long time, and even until decomposi- 

 tion had set in : this is a curious fact, which seems to me to be 

 evidently in connexion with the prevision which presides over 

 the conservation of species. 



Note. — Since writing'this memoir, I foand, on the 8th of June 

 1863, on the gills of Merluccius vulgaris, a female Chondracan- 

 thusy to which two male individuals, arrived at their perfect de- 

 velopment, were attached by a frontal appendage. This evidence 

 seems to me to convert the hypothesis above proposed by me 

 into a certainty, and to confirm my supposition that the males, 

 for the purpose of propagation, attach themselves artificially to 

 the females by the singular means which I have described. 



18* 



