VIII AETHEOPODA 207 



as atypical, but a long convoluted glandular tube lies in the mid- 

 dorsal region of the head and opens at the front end of the carapace, 

 and this tube secretes a long gelatinous filament. 



The larval stage of the Copepodid only lasts thirty-six hours at 

 most. The larva is swallowed by the rock-bass, and has the instinct 

 to burrow into the mucous membrane of the pharynx of its host. 

 When the pointed front end of the carapace of the parasite comes 

 into contact with the bone of the gill arch of the host, the distal end 

 of the filament already referred to is extruded from the frontal 

 gland, and adheres thereto. The larva backs off, and the filament 

 draws out ; but before it is completely extruded the larva grasps the 

 end of it by the incurved hooks of the second maxillae, and holds on. 



A moult follows, in which the maxilbpedes are shifted forward 

 so that their bases are now 

 situated between those of 

 the second maxillae. The 

 second maxillae are greatly 

 enlarged, and have lost their 

 segmentation though their two 

 incurved ends still tightly 

 grasp the filament. A sucking 

 tube is formed by the union of 

 a projecting labrum with an '"^ 



under lip, and inside this are Fig. 1 5 4.— Enlarged view of gnatlntes and lips 



the mandibles, though they can "■^''"'f Lf ''^wf '"f^°^'''''^ '^™ ^""^ *'' 



, .ft f ft -I J J 1 1 S1U6. I AitGr Wilson, ) 



stiU be forced out through 



lateral slits between these lips. "^'''"^ "^ '" ""ZT^ln"- '" '''"""' 



The minute first maxillae are 



attached to the sides of the sucking tube, and the two pairs of 



antennae are much shortened; they are reduced in fact to mere 



stumps (Fig. 154). 



The animal can now stab the vascular giU of its host with its 

 needle-like mandibles and suck its blood, and it grows rapidly in 

 size, moulting frequently. At the first of these moults practically 

 the adult form is attained, all trace of the thoracic appendages is 

 lost, and the maxillipedes are transformed into blunt shapeless lobes. 

 The filament of the female shortens till the claws of the second 

 maxilla are actually in contact with the skin of the host ; that of the 

 male, however, remains long, and he appears to crawl aroixnd like a 

 tethered goat until he finds a female; then he relaxes his hold on 

 the filament and seizes the female with his claws, and so is in a 

 position to effect sexual union. 



An examination of the differences between the Copepodid larva 

 and the typical Copepod shows examples of all the changes we 

 postulated in explaining the Nauplius ; — we have the reduction in 

 segmentation, and the disappearance of appendages, or rather the 

 replacement of a homologous series of appendages by a smaller 

 number of similar ones; in fact there is a functional rather than 



