68 



L WER IN VEll TEBRA TES. 



the last year's skeleton as if it were a treUis, which, when once constructed, could be 



used repeatedly. 



Tliis is not hard to believe, since two branches of the same sponge will unite it 



brought in contact, and even two sponges 

 of the same species will not infrequently 

 combine to form a single specimen. A 

 certain proportion of some of the fresh- 

 water sponges does outlast the winter, 

 and these old skeletal frames frequently 

 contain many statoblasts. 



Certain of the fresh-water sponges in 

 tropical countries have to pass through 

 a dry season, and it is supposed, with a 

 considerable amount of probability, that 

 their statoblasts can undergo dessication 

 without loss of vitality, and even that 

 they may be carried by the winds, thus 

 affording the starting points for colonies 

 in new localities when the rainy season 

 sets in. Some forms are described as 

 having no statoblasts. 



Oedee II. — SILICOIDEA. 



This, the highest order of the sponges, 

 is characterized by having the skeleton 

 almost entirely composed of silicious 

 spicules. 



Stjb-Oedbe I. — Teteactinellin^. 

 This group can be represented by 

 Tethya, in which the skeleton is radia- 

 tory. The typical spicules have a long, 

 straight axis and three curved arms, 

 reminding one of an anchor, or more 

 accurately, a grapnel. There are also 

 long, straight spicules with both ends 

 alike, and star-shaped silicious bodies. 

 By the latter these sponges are allied 

 to the Gummininee. Geodia is another 

 remarkable type in this group, with ex- 

 tremely thick and unusually large in- 

 ternal spicules. When dried, these 

 sponges are as hard as if carved out of wood. According to Zittel, the greatest 

 authority on fossil sponges, this sub-order first appeared in the carboniferous, but 

 was represented only by isolated spicules until the genus Geodia appeared in the 

 Jurassic. 



Fig. 58. - EupUctella aspergUlum, Venus flower-basket. 



