154 



LECTURE XIII. 



and generalised by the careful observations of Dr. Nordmann. The 

 following is the general 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 

 membrane is thinner, and includes both the vitelline mass and albu- 

 men. The yoke 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. 83. ci). 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 slightly elongates ; the 

 exterior albuminous fluid inceases^ the inner mem- 

 brane expands^ and the outer one bursts and is shed. 

 The movements of the imprisoned embryo increase 

 in force until it bursts the remaining membrane of the ovum and 



escapes from the ovigerous sac. It 

 then presents the form represented in 

 figure 84. ; the digestive sac (Ji) is now 

 '^Z/ '^]Sr'"^j!| ^^ discernible, together with a peculiar tor- 



tuous tube (^), which is continued from 

 the eye-speck. In the course of half an 

 hour the young Achtheres undergoes its 

 second stage : the first integument is 

 loosened by the formation of a second beneath it, which now incloses 

 a body, altered in its shape and in the number and 

 nature of its appendages. 



The process of moulting lasts from eight to ten 

 minutes. The body {fig. 85.) is divided into an 

 anterior and a posterior segment, the latter con- 

 sisting of four joints. A pair of four-articulate se- 

 tigerous antennae diverge from the anterior part 

 of the body. Between the antennae is the large 

 single median eye, as in the monocular Entomo- 

 stracous Crustacea. The little Epizoon is now pro- 

 vided with five pairs of feet, the first three pairs 

 terminate by a simple hook ; the last two pairs are 

 bifurcated, one division being hooked and prehensile, the other 



