274 LECTURE XIII. 



LerntBocera, still in the ovum, which, instead of resembling the 

 parent, presented the characters of a locomotive Entomostracous 

 monoculous Crustacean. 



The singular metamorphosis thus indicated has been traced out and 

 generalised by the careful observations of Dr. Nordmann. The 

 following is the course of development of the Lernaean parasite of the 

 Perch. 



The female Achtheres is devoid of ovigerous appendages in the 

 months of December, January, and February. In March they are 

 developed by the eversion of a membrane prepared in the ovarian 

 sac. Each sac hangs by a short tubular peduncle which is in direct 

 communication with the short oviduct. The outer membrane of the 

 ovum or chorion is moderately thick and transparent ; the inner mem- 

 brane is thinner, and includes both the vitelline mass and albumen. 

 The yolk forms the largest proportion of the contents of the ovum, 

 and is finely granular. One of the first parts of the embryo discerned 

 by Nordmann was the dark ocellus {fig. 118, e,). A pair 

 of cylindrical processes shoot out from each side of the 

 fore part of the embryonic or vitelline mass ; and a 

 pencil of hairs is developed from the extremity of each 

 process. The body (d) slightly elongates, the exterior 

 Achtheres. Egg albumiuous fluid (c) increases, the inner membrane {b) 

 and Embryo. Q^pands, and the outer one {a) bursts and is shed. The 

 movements of the imprisoned embryo increase in force until it bursts 

 the remaining obstacle, and escapes from the ovigerous sac. It then 

 119 ,^s^^^ presents the form represented in figure 

 119. The digestive sac {Ji) isnow discerni- 

 ble, together with a peculiar tortuous tube 

 {g'\ which is continued from the mouth. 

 The locomotive organs are in two pairs, 

 and consist of tubular processes of the 

 Achtheres: first stage. intcgument, including a fasciculus of 



bristles ; they now present the type of those of the Anellids. And 

 since we were led from the Infusoria to the Polypi, because the cili- 

 ated larvae of these resembled the monads, and from the Polypi to 

 the Acaleph^, because these in their larval state were polypes, so we 

 have now the same indication, from a transitory step in development, 

 of the right track in passing from the Annulata to the Epizoa ; and 

 the succeeding steps will lead us to place these parasites on a higher 

 grade of articulate structure ; and not with the Entozoa, where 

 Cuvier and Lamarck left them. 



In the course of half an hour the young Achtheres undergoes its 

 second stage : the first integument (e) is loosened by the formation of 



