292 THE ANATOMY OF INVERTEBRATED ANIMALS. 



of a plate, free at the outer end. This plate, in all the mem- 

 bers, except the ophthalmic peduncles and the mandibles, 

 then becomes bilobed externally, the inner lobe representing' 

 the endopodite, while the outer is the representative of the 

 exopodite and epipodite. The two latter, when they are in- 

 dependently developed, become separated by the division of 

 the outer lobe. The gills arise partly as outgrowths from 

 the epipodites, partly as distinct processes from the parts to 

 which they are eventually attached. The division of the 

 limbs into articulations takes place from their distal toward 

 their proximal ends. The heart appears late, at the posterior 

 extremity of the cephalo-thorax, and therefore behiud the 

 yelk-sac. 



The nervous system of the post-oral portion of the ceph- 

 alo-thorax consists at first of eleven pairs of ganglia, cor- 

 responding with the mandibles, maxilla?, maxillipede^, and 

 ambulatory legs. The six anterior post-oral ganglia of each 

 side soon coalesce in pairs, so as to form as many single gan- 

 glia ; and of these the four anterior, namely, the mandibular, 

 the two maxillary, and the first maxillipedarv ganglia, unite 

 into a single mass ; the two hinder ganglia, that is to say, 

 those of the second maxillipedary somite, next coalesce in 

 the same way, and it is only subsequently that the two masses 

 thus formed become fused into the single anterior post-oral 

 ganglion of the adult. The other ganglia not only remain 

 separate, but become wider apart with advancing age. A 

 ridge on each side of the oesophagus at first represents the 

 cerebral ganglion and the commissural cords, the latter being 

 developed out of the posterior part of the ridge, and the for- 

 mer from its anterior portion. The cerebral ganglia are at 

 first two on each side, but the posterior, whence the nerves to 

 the antennary organs proceed, is much larger than the other, 

 and would appear to represent two ganglia. The endoster- 

 nites arise as processes from each of the eight posterior ceph- 

 alo-thoracic sterna, which eventually arch over the gangli- 

 onic cord, and unite with one another. 



The alimentary canal is produced by the gradual differen- 

 tiation and demarcation of the sternal part of the hypoblast, 

 which invests the whole yelk, from the tergal part, which be- 

 comes the yelk-sac. 1 



1 According to Bobretsky (/. c.) there is no proper yelk-sac, the structure so 

 termed by Rathke being the saccular hypoblast, which is formed by invaerina- 

 tion of the primitive blastoderm, and encroaches upon the vitellus, until the 

 latter is all absorbed. The hypoplastic sac is converted into the liver and the 

 intestine. The stomach arises independently by invagination of the epiblast. 



