POLYZOA. 405 



the haematoxylin was prepared from fresh logwood or not, that 

 which he had employed was bought, and therefore he could not 

 say how it had been prepared. He had obtained samples of 

 haematoxylin from several different places, and where the crystals 

 were well formed, he had not found the slightest difference in the 

 result. Whether there was any difference in haematoxylin prepared 

 from fermented or unfermented logwood, he could not say. 



By W. G. VVheatcroft. Plates XVIIL— XIX. 



POLYZOA are members of the sub-kingdom Mollusca. 

 This sub-kingdom is divided into two great divisions : — ■ 

 MoLLUSCOiDA and the Mollusca proper. The nervous 

 system of the former of these consists of a single ganglion, or 

 principal pair of ganglia, and there is no circulatory organ, and 

 an imperfect heart. Allman describes the members of the class 

 Polyzoa as follows : — " Alimentary canal suspended in a double- 

 walled sac, from which it may be partially protruded by a process 

 of evagination, and into which it may be again retracted by inva- 

 gination. Mouth surrounded by a circle or crescent of hollow, 

 ciliated tentacles ; animals always forming composite colonies." 



All the Polyzoa live in an associated form in colonies, some- 

 times branched and plant-like, as in the Sea Mat {Fliistra foliacea) 

 (Plate XVIIL, Fig. i), sometimes encrusting, and very rarely are 

 free. " Each polyzoarium," to quote Prof. Alleyne Nicholson, 

 " consists of an assemblage of distinct but similar zooids arising 

 by continuous gemmation from a single primordial individual. 

 The colonies thus produced are in very many respects closely 

 similar to those of many of the Hydroid Polypes, with which, 

 indeed, the Polyzoa were for a long time classed. 



The Polyzoarium, however, of a Polyzoon differs from the 

 polypidom of a composite hydroid in the general fact that the 

 separate cells of the former do not communicate with one another 

 otherwise than by the continuity of the external integument, 

 whereas the zooids of the latter are united by an organic connect- 



