374 



1st. The skeleton. Its general structure and component parts. 

 Under this head the physiological purposes of the various forms of 

 spicula, treated of in the first part of the paper, are described, and 

 their peculiar offices in the sponge pointed out. 



2nd. The sarcodous system is considered by the author as the 

 homologue of the mucous lining of the stomach and intestines of the 

 higher tribes of animals, and probably as the equivalent of the nervous 

 system also. 



3rd. The interstitial canals are considered as the equivalents of 

 the stomach and alimentary canals of the higher animals. 



4th. The intermarginal cavities, situated immediately beneath the 

 surface, and receiving the incurrent streams from the pores, are be- 

 lieved by the author to be the organs for the secretion of the vital 

 fluids of the animal. 



5th. The dermal membrane, enveloping the whole of the sponge, 

 and in which the inhalant and exhalant orifices of the animal are 

 situated. 



6th. The pores or inhalant orifices. These organs are not per- 

 manent ; i. e. they are opened and closed at the will of the animal, 

 and when once closed seldom occur again in precisely the same spot. 



7th. The oscula or excurrent orifices, usually permanent organs, 

 and capable of being opened or closed in accordance with the neces- 

 sities of the animal. 



8th. Inhalation and exhalation. Two modes of these operations 

 are described ; one as occasional or intermittent, but very powerful 

 for the imbibition of nutriment ; the other gentle and continuous for 

 the purposes of the aeration of the vital fluids, and for the ejection 

 of digested matters. 



9th. Nutrition. The modes of imbibition and periods of digestion 

 are treated of, and the author describes a series of contrivances by 

 which some sponges are in possession of peculiar organs which en- 

 able them to prey upon annelids or other soft creatures that may 

 crawl over their surface or intrude within their cavities or canals. 



10th. Cilia and ciliary action. The accounts of the cilia of the 

 gemmules or ova, as described by Dr. Grant and other writers, are 

 referred to by the author ; and the same organs in situ in Grantia 

 compressa are pointed out as the powers on which inhalation and ex- 

 halation are dependent. 



