ENTOMOSTRACA, HISTORY OF DEVELOPMENT. 547 



emersion from the egg differ considerably from the parent, espe- 

 cially in having only the thoracic portion of the body as yet 

 evolved, and in possessing but a small number of locomotive ap- 

 pendages ; the visual organs, too, are frequently wanting at first. 

 (See Fig. 276, C-G.) The process of development, however, 

 takes place with great rapidity ; the animal at each successive 

 moult (which process is very commonly repeated at intervals of 

 a day or two) presenting some new parts, and becoming more 

 and more like its parent, which it very early resembles in its 

 power of multiplication, the female laying eggs before she has 

 attained her own full size. Even when the Entomostraca have 

 attained their full growth, they continue to exuviate their shell 

 at short intervals during the whole of life; and the purpose 

 which seems to be answered by this repeated moulting, is the 

 preventing the animal from being injured, or its movements ob- 

 structed, by the overgrowth of parasitic Animalcules and Con- 

 fervse ; weak and sickly individuals being frequently seen to be 

 so covered with such parasites, that their motion and life are 

 soon arrested, apparently because they have not strength to cast 

 off and renew their envelopes. The process of development ap- 

 pears to depend in some degree upon the influence of light, being 

 retarded when the animals are secluded from it ; but its rate is still 

 more influenced by heat; and this appears also to be the chief 

 agent that regulates the time which elapses between the moult- 

 ings of the adult, these, as in the Daphnia, taking place at inter- 

 vals of two days in warm summer weather, whilst several days 

 intervene between them when the weather is colder. The cast 

 shell carries with it the sheaths not only of the limbs and plumes, 

 but of the most delicate hairs and setse which are attached to 

 them. If the animal have previously sustained the loss of a 

 member, it is generally renewed at the next moult, as in higher 

 Crustacea. 1 



372. Closely connected with the Entomostracous group is the 

 tribe of Suctorial Crustacea; which for the most part live as 

 parasites upon the exterior of other animals (especially Fish), 

 whose juices they imbibe by means of the peculiar proboscis- 

 like organ which takes in them the place of the jaws of other 

 Crustaceans; whilst other appendages, representing the feet- 

 jaws, are furnished with hooks, by which these parasites attach 

 themselves to the animals from whose juices they derive their 

 nutriment. Many of the Suctorial Crustacea bear a strong re- 

 semblance, even in their adult condition, to certain Entomo- 

 straca ; but more commonly it is between the earlier forms of 

 the two groups that the resemblance is the closest, most of the 

 Suctoria undergoing such extraordinary changes in their progress 

 towards the adult condition, that if their complete forms were 

 alone attended to, they might be excluded from the class alto- 



1 For a complete and detailed account of this group, see Dr. Baird's " Natural History 

 of the British Entomostraca," published by the Ray Society. 



