EPIZOA. 153 



seen at the middle line of tlie cephalo-thorax propelling the blood 

 forwards by rythmical contractions. Two canals (??, n) pass from 

 it into the hollow prehensile feet. The rest of the blood is dis- 

 tributed to the head, and along each side of the commencement of the 

 alimentary canal to the under part of the body, where it passes back- 

 wards in the vessel which accompanies the intestine. 



The ovaria (o, o') at first appear in the form of a slightly flexuous, 

 long, blind tubes, sacculated along one side. As the ova are developed, 

 the ovarium takes on the form of a bunch of grapes, and occupies the 

 whole cavity of the abdomen external to the intestine : each ova- 

 rium terminates by a triangular, and somewhat prominent orifice, 

 to which the external ovisac (/) is appended. 



In the minute male {fg. 82.), the testes are indicated by four dark- 

 coloured and finely granulated bodies situated in 

 the posterior segment or abdomen. 



The first remarkable circumstance in the natural 

 history of the aquatic Epizoa is the constancy with 

 which particular species infest particular fishes or 

 Crustacea. And how, it may be asked, can creatures 

 so devoid of means of transport, nay, in most in- 

 stances, of the power of detaching themselves from 

 Achtheres, male, the auimals wheucc, like foetuses, they derive their 

 magni ed. mcans of growth, originally reach the precise species 



of animal and organ to which they are habitually attached ? 



Are certain of the ova accidently retained near the parent after 

 the rupture of the ovisac, and there grow, like seeds of plants fallen 

 in a favourable soil ? Or, do some of the liberated ova, by a happy 

 fortuity, arrive at the appropriate organ of the appropriate species? 

 and are they there accidentally retained until the prehensile instru- 

 ments are developed ? Such hypotheses may be permitted in reference 

 to the ova of an Entozoon which are developed by millions, and need 

 only to be swallowed by the animal in whose intestine they are 

 adapted to exist, but the ova are too few in the Epizoa, and the parts 

 to which they are attached are too exposed, to allow of the suppo- 

 sition that their parasitic growth is dependent on such accidental 

 circumstances. M. M. Audouin and Edwards appear to have 

 been the first to suggest that the sedentary Lernaean Epizoa might 

 enjoy at a previous period of existence locomotive powers, and the 

 hypothesis was supported by the discovery, made by Dr. Surriray, of 

 the embryo of a Lernceocera, still in the ovum, which, instead of re- 

 sembling the parent, presented the characters of a locomotive 

 Entomostracous monoculous Crustacean. 



The singular metamorphosis thus indicated has been traced out 



