88 SPONGES 



tube, which act as valves respectively, the former favouring an inflow and 

 hindering an outflow, tlie latter having a contrary action. The beats of 

 the flagella cause alternating, negative, and positive pressures in the interior 

 of the canal system ; the former cause water to flow in at the pores, the 

 latter result in its ejection at the osculum. "When the current is once well 

 started it draws, like a flue, and so favours its own continuance, its action 

 being comparable to the fly-wheel of a machine. Closure of the pores at 

 once stops the current, without, however, causing any pressure in the 

 interior, wliich would be dangerous to delicate tissues. The irregular beats 

 of the flagella then simply cause eddies and vortices in the gastral cavity 

 or chambers. 



Bionomics and Natural Hidorij. — Sponges have a wide range of habitat 

 and are found living under the most varied conditions of existence, from 

 the shore-line, where they are continually subjected to most violent 

 stresses and strains, down to the calm and placid environment of the 

 ocean abysses. The influence of these difi"erent life conditions is seen 

 especially in the body form and in the skeleton. Sponges living on mud 

 or ooze show a further adaptation in the form of an anchoring root tuft 

 (see above, p. 3). Fresh-water sponges require to be able to withstand 

 greater vicissitudes than marine forms, whose environment, however 

 boisterous, is more uniform. As an adaptation to life in fresh water we 

 may mention the gemmules already described. Many siliceous sponges, 

 belonging to families far apart in the system, have the power of excavat- 

 ing calcareous rocks or shells to form tunnels which they inhabit. The 

 Clionidae are the best known instances of this. It is not clear how the 

 perforation is eff'ected. The sponge may in later life grow out of its 

 excavations and become simply an incrusting or massive form of the 

 ordinary type. 



Animals so full of cavities as are sponges offer a shelter to many 

 other creatures, some of which are always found as commensals of sponges ; 

 as instances we may mention various Crustacea, e.g. Typton, Spongicola, 

 and Hydrozoa, e.g. SpoiKjicola Jistularis, F.E.S. ( = Stephanoscyphus mirahilis, 

 Allman), found in EnjJerella, and Authozoa, e.g. Pahjthoa (Figs. 19, 24). 

 Sponges themselvis appear to be very distasteful to other animals and 

 are eaten by very few. Some Nudibranch.'^, however, fied on them 

 and may then mimic closely the sijonges upon which they feed ; as 

 instances of this we may mention Jorumia Johnstoni, which feeds on 

 Halichondria, and Rostanga coccinea, which lives upon red incrusting 

 sponges. Both these Nudibranchs resemble the sponges upon which they 

 respectively live, both in colour and in surface texture (see Garstang, 

 Concholoijist, ii. 3 (1892) ; and Joum. Mar. Biol. Ass. iii. 3, p. 22u). 



The distaste fulness of sponges often leads to a symbiosis between 

 them and otlier animals, especially crabs. Suberitcs commonly grows on 

 the shells of hermit crabs, and soon absorbs the shell, so that the crab 

 inhabits a cavity in the sponge. Other crabs cover themselves with bits 

 of sponge wliich they plant on their carapace, on which the sponge grows 

 and moulds itself. It is very jirobable that the disUisteful and highly- 

 smelling sponge protects the crab from the atUicks of tisli or ceplialopods, 

 imparting to it, as it were, its own qualities. 



