PALINURUS OR THE SPINY LOBSTER OF BOMBAY. 385 



actually see take place, the separate thoracic ganglia of the larva, 

 rapidly closing up and amalgamating in the pupa in the course of a 

 few hours. 



We have now learned that in the course of development of the 

 individual Palinurus, the separate ganglia of the somites of the larva 

 coalesce to form the large ganglion of the adult. We have also seen 

 that in the " lower " earlier or more primitive type of Crustacean 

 like the Sand-hopper the ganglia of each somite are separate and 

 paired as is also the connecting cord. We have in the higher Lobster 

 a further degree of concentration, the six ganglia behind the mouth 

 having coalesced. In the Prawn, Palajmon, we have a stage inter- 

 mediate between the Lobster and Palinurus, as the ganglia of the 

 post-oral somites of the cephalo-thorax have all united, but the union 

 is indicated by distinct indentations. Then above Palinurus we have 

 in the Crab all seventeen post-oral ganglia united into a single mass 

 with practically no indication of metamerism though in the larva it is 

 distinct. 



Now in Mr. Darwin's theory of the origin of species it is assumed 

 that the higher types such as the Crabs have developed through past 

 ages by a succession of stages through ancestors resembling or common 

 to such lower animals as Palinurus, Prawn, Lobster and Sand- 

 hopper. In seeking to explain the origin of any species you will be 

 greatly helped by the rule of which we have now seen an excellent 

 example, that the course of the devel >pment of an individual from the egg 

 through its embryonic or larval stages up to that of the. adult is an epitome 

 of and furnishes a clue to the history of the development of the species. 



The Reproductive Organs. 



These have been already described with the topographical 

 anatomy of the cephalo-thorax. It should be further noted that the 

 gonads are hollow organs continuous with their ducts. You have 

 seen in the Earthworm, Frog and Rabbit types that the ova are dis- 

 charged into the ccelom and taken thence to the exterior by their ducts. 



You have seen in Palinurus the body-cavity is not lined with 

 epithelium as in these three types, but is continuous with the cavity of 

 the blood-vessels. It is therefore not homologous with the ccelom, and 

 may be called the hcemocosle (Gr. haima, blood, koiloma, a cavity). 



