178 anato'my and physiology 



disposed at nearly right angles to the surface. Oscula 

 congregated, elevated on numerous long fistulse. Fis- 

 tulae composed of numerous parallel fasciculi, radiating 

 from the base to the apex of each in straight or shghtly 

 spiral lines. 



Type, Polymastia mammillaris, Bowerbank. 



This genus is closely allied to Alcyoncellum, Quoy et 

 Gaimard ; the principal difference being that in the latter 

 the sponge always consists of a single fistula, while in the 

 former it is constructed of a basal mass from which nume- 

 rous fistulse emanate. The form and structure of the 

 fistular organs in each genus very closely resemble each 

 other. Beside these structural differences, there are others 

 of a less striking description, that strongly indicate the 

 necessity for generic sej)aration. Thus in Alcyoncellum cor- 

 bicula, in the Museum at Paris, and Euplectella aspergillum, 

 Owen, there are an abundance of interstitial spicula of rec- 

 tangulated hexradiate forms, which are very characteristic 

 of those species, while the British species of Polymastia 

 with which we are acquainted appear to be totally destitute 

 of these complicated and beautiful forms of spicula. I have 

 therefore thought it desirable, notwithstanding the close 

 agreement that exists in the structure of their fistulse, that 

 a generic distinction should be established between them. 



Halichondria mammillaris, Johnston, is the best type of 

 the genus Polymastia. Tho whole of the parietes of these 

 elongated fistulse are inhalant. In some specimens of P- 

 mammillaris dredged in Vigo Bay by my friend Mr. 

 McAndrew, the open pores are exceedingly numerous, and 

 the exhalant organs are as distinctly shown to be confined 

 to the distal extremities of the fistulse. 



Kg. 358, Plate XXIX, represents a view of a small 

 portion of the side of one of the large cloacae of Polymastia 

 robusta, Bowerbank, exhibiting the structure and mode of 

 disposition of the longitudinal skeleton fasciculi, X 25 

 linear. 



