338 LECTURE XV. 



anterior and posterior divisions of the body: blood-vessels seem 

 to be prolonged from it, and its pulsation speedily becomes dis- 

 tinguishable. The nervous system consists at first of two parallel 

 lines, on each of which are eleven minute white spots : from the 

 anterior of these a short and broad process passes forwards on either 

 side of the oesophagus. The above described stages of development 

 are in progress from the beginning of April to the middle of May. 

 The whole of the organs continue to approach more nearly to their 

 mature form. The brain, liver {v\ and salivary glands next make 

 their appearance. The outer integument of the body is developed 

 from the ventral to the dorsal aspect, and the yolk-laden intestine is 

 finally, with the heart, Availed in by the confluence of the lateral 

 lobes of the integument along the middle line of the back. 



The integument is very soft when the animal quits the shell: 

 the young craw- fish subsists, at first, on the remaining portion of 

 the yolk, during which time its coat becomes sufficiently hardened 

 to admit of its moving about in quest of food with more safety. 

 The different appendages increase in length, and more especially 

 the branchiae, the growth of which is now remarkably rapid. 

 The changes of the interior parts of the animal, with the exception 

 of the development of the sexual organs, consist in a gradual 

 adaptation of parts already formed to their proper functions. The 

 relative positions of embryo and vitellus are the same in the 

 craw-fish as in the Daphnia pulex and Branchipus stagyialis. The 

 maxillae present at an early period a considerable resemblance to 

 those of the Apus ; the legs, at the period when they are devoid of 

 branchial appendages, typify the persistent condition of the Bran- 

 chiopoda; after the branchiae are developed, and before they are 

 enclosed in the branchial chamber, the characteristic persistent con- 

 dition of the respiratory system of the Edriophthalma and Stomapoda 

 is sketched out. 



M. M. Audouin and Milne Edwards have shown that the successive 

 changes of development of the nervous system of the craw-fish cor- 

 respond in like manner with distinct types of formation observed by 

 them in its permanent condition in lower species of the class ; thus 

 the two series of ganglions, which first indicate the suboesophageal 

 central part of the nervous system in the embryo craw-fish, are ana- 

 logous to the permanent state of the nervous system in the mature 

 Talitrus. At a more advanced period, the two series of ganglions 

 in the foetal craw-fish approach the median line and become united 

 together as in the abdominal ganglionic chain in the adult Cymothoa. 

 We have seen, that in the brachyurous Crustacea a further con- 

 centration takes place by the longitudinal blending together of the 



