22 



The interior of this sac is the simple digestive cavity or 

 Btomach, into which the food is received, and after under- 

 going the digestive process, the debris or insoluble portions 

 are ejected. This gelatinous sac is endowed with a re- 

 markable power of contractibility, by means of which it 

 can change its shape, rapidly contracting into a globular 

 form when irritated or alarmed, and again expanding into 

 the long cylindrical shape here shown. The tentacles arc 

 equally endowed with this contractile property, by means 

 of which they seize and close around their prey ; and they 

 are moreover gifted with a remarkable power of benumbing 

 and killing their victim while struggling in their grasp ; 

 which faculty resides in certain minute poisonous darts, 

 which they can project with great force, and which aro 

 called nettling threads. The marine species are for the 

 most part more complex than this, being compound struc- 

 tures, consisting of an aggregation of such simple sacs as 

 that I have described, united together by a common trunk, 

 so as to give a branched and plant-like aspect to the ani- 

 mal, as shown in D 35 and others. These compound forms, 

 called Hydroid zoophytes, from their being formed on the 

 type of the hydra before described, are invested 

 with a horny tube, which they have the power of secreting 

 to serve for protection and support, highly requisite in the 

 boisterous element they inhabit. This horny sheath called 

 a polypidon invests the stem and branches, and provides 

 cup-like cells, of various shapes, into which the polyp-heads 

 can contract and conceal themselves when danger threatens. 

 It varies in size, with the contained polyp, from a few line 

 to a foot in height ; is for the most part erect and shrub- 

 like, attached by root-like fibres to stones and shells, or 

 sometimes creeping like ivy over sea-weeds and rocks. The 

 genera of the Hydroid zoophytes are founded upon the con- 

 figuration and degree of branching of this investing sheath, 

 or polypidon as it is termed, together with the form and 

 arrangement of the cells : having premised which, I will 

 proceed to enumerate the shor« species." Dr. Pittock then 



