ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 625 



Hamann considers as a schizocoel, into the subambulacral cavities, 

 and into the general cavity. 



In star-fishes, then, as in Echinids and Comatulids, sea water plays 

 an important physiological part, but its course is not regulated by as 

 complicated a system of irrigating canals ; this leads to a division of 

 the Echinodermata into two great groups, one of which contains the 

 Cystoidea, Blastoidea, Stellerida, and Ophiurida, and the other the 

 Crinoids, Echinoids, and Holothnrians. In this phylum, as in Coelen- 

 terata and sponges, the penetration of water is a general phenomenon, 

 while it is rare in worms, Arthropods, Mollusca, and Vertebrata ; we 

 may, therefore, with de Blainville, divide animals into the three great 

 groups of Protozoa, Phytozoa, and Artiozoa. 



Vascular System of Spatangus purpureus.* — M. H. Prouho 

 ascribes the difficulties in homologizing the vascular system of Spa- 

 tangids with that of regular Echinids to the imperfect observation of 

 certain anatomical facts. He finds that the two vascular systems of 

 Spatangus are as distinct as in Echinus, and their relations are exactly 

 the same. The only difference is that, instead of there being a double 

 Polian ring as in Cidaris, there is a double Polian canal. The sand- 

 canal and the ovoid gland have exactly the same relations in Spatangus 

 as in the regular forms. What has been called the sand-canal in 

 Spatangus is really the homologue of the Polian ring of the Cidaridje, 

 and it is therefore proposed to call it the Polian canal. The term 

 sand-canal or aquiferous tube must be reserved for the vessel which 

 extends from this double canal to the posterior extremity of the 

 madreporite apophysis. 



Ccelenterata. 



Origin of Metagenesis in Hydromedusse.t — Mr. W. K. Brooks 

 considers that the view usually held, viz. that the sessile colony is 

 the primitive form, from which medusae have been derived by division 

 of labour and the specialization of the reproductive member of a 

 polymorphic hydroid corm, is irreconcilable with the life-history of 

 Narco- and Tracho-medusaa. 



In Liriope amongst the latter, and in JEginata and Cunina octo- 

 naria amongst the former, a true planula, and a true hydra stage is 

 passed through which developes directly into medusas. " The life- 

 history of these forms proves conclusively that the medusa stage is 

 older than the sessile hydroid-corm, which has arisen through the 

 power to multiply asexually, which is possessed by the hydroid larva 

 of the medusa." 



By means of diagrams the life-history of various types is shown. 

 Commencing with the above simple life-history, through that of 

 C. parasitica, in which the actinula, or floating hydra, never becomes 

 a medusa, but buds off hydraa which thus develope, he passes to the 

 still more complicated instance of Turritopsis. Here the planula, 



* Comptes Rendus, cii. (1886) pp. 1498-1500. 

 t Johns-Hopkins Univ. Circ, v. (1886) pp. 86-8. 

 Ser. 2.— Vol. VI. 2 T 



