Polyps, Jelly-fishes and Sponges. 



69 



commerce belongs to a quite different, altliough allied, division. 

 Here the coral poh'ps differ from those of the Madrepores and 

 from the sea-anemones in having the tentacles branclied and 

 limited to a constant number — eight. 



The precious coral is an inhabitant of the i\'Iediterranean 

 and other warm seas, but an ally occurs in our fauna, Alcyoniuni 

 digitatum, popularly known as Dead IMan's Fingers (Fig. 83). 

 Colonies of this form are often washed up on the beach, and when 

 seen in such a situation appear as dirty yellow, almost shapeless 

 masses of a leather}' texture, provided with blunt finger-shaped 

 processes. Nothing could be more unattractive to the eye, 

 yet in the living, expanded condition these colonies are objects 

 of great beaut}', and are seen to consist of innumerable little 

 white polyps bound together b}-' a gelatinous-looking semi- 

 transparent substance, which acts as a kind of skeleton. The 

 polyps of Alcyonmni have eight delicately branched tentacles, 

 and their tissues, as well as 

 the gelatinous skeleton, are 

 strengthened by little sharp 

 spicules, slender rods of a hard 

 matter, which are possibly of 

 use in rendering the tempting- 

 looking colonies unpalatable 

 to predaceous animals, which 

 would otherwise devour them. 



Although, on account of 

 their plant-like appearance, 

 often mistaken forseaweeds,the 

 delicately branched Zoophytes 

 are also colonies composed of numerous little polyps, often so small 

 as to be visible only under the microscope. It is, indeed, to 

 one of these that the term poh'p was first applied, on account of 

 a fanciful resemblance between this little animal and a diminu- 

 tive Octopus (Polypus of the Greek). These Zoophytes, also 

 called Hydroids, are extremely abundant, and occur in many 

 different situations ; in rock-pools they usually form merely 

 fir-hke growths on stones or shells ; but the deeper water species, 

 the colonies of which are often cast up on the beach, may attain 

 a height of several feet, and branch most luxuriantly. 



A typical Zoophyte is the common Ohelia gemcukiia (Figs. 84 

 and 85) so often found growing on the broad fronds of laminarian 

 weeds. The colony here consists of a number of branched fila- 

 ments, about the thickness of ordinary sewing-thread, which are en- 

 cased in a skeletal tube composed of a flexible, horny substance, 



FIG." 85. OBELIA GENICULATA 



(after HINCKS). 



