478 



POLYZOA 



Membranipora membranacea commonly covers many square 

 inches of the frond of Zaminaria with its delicate lace-lil 

 encrustation. Mtsche l has shown that this species has ite 

 calcareous matter deposited in plates, separated by intervals of 

 uncalcified ectocyst. The effect of this arrangement is to make 

 the colony flexible, and to enable it to adapt its shape to the 

 movements of the Zaminaria, which is swayed to and fro by 

 the action of the waves. Many of the calcareous forms grow- 

 ing on Zaminaria have no special arrangement of this kind, 

 and they accordingly grow in colonies whose area is so small 

 that the greatest movements to which the seaweed is liable are 

 not sufficient to crack or break the colony. 



Many species show a decided, or even exclusive, preference 

 for particular situations; as, for instance, species of Triticella, 

 which are only found on certain Crustacea. Many encrusting 

 forms prefer the inside of dead shells of Pecten, Cyprina, etc., to 

 any other habitat. Terebripora 2 excavates tubular cavities in 

 the substance of the shells of Molluscs. Hypophorella 3 inhabits 

 passages which it forms in the walls of the tubes of the Poly* 

 chaets, Zanice and Chaetopterus. Zepralia foliacea, one of the 

 Cheilostomata, forms masses which may reach a circumference of 

 several feet, simulating a small coral-reef. Its contorted plates are 

 a regular museum of Polyzoa, so numerous are the species which 

 delight to find shelter in the quiet interstices of the colony. The 

 exquisite little colonies of Crisia ebumea are commonly found on 

 red seaweeds, or on the branches of the Hydroid Sertularia. 



The Polyzoa are found at all depths, certain Cheilostomes 

 having been recorded from 3000 fathoms. The Cyclostomes 

 dredged by the "Challenger" were all found in depths of 1600 

 fathoms or less, while the Ctenostomes are a distinctly shallow 

 water group, most having been found at less than 40 fathoms, 

 and only three at so great a depth as 150 fathoms. 4 



A few forms {Membranipora pilosa, Scrupocellaria reptans, etc.) 

 are known to be phosphorescent ; 5 but it is not known what is 

 the purpose of this phenomenon. 



1 Zeitschr. wiss. Zool. xxi. 1871, p. 421. 



2 Fischer, Nouv. Arch. Mus. Paris, ii. 1866, p. 293. 



3 Ehlers, Abh. Ges. Gbttingen, xxi. 1876, p. 3, and Joyeux-Laffuie, (as Dclagia) 

 Arch.. Zool. Exp. 2 ser. vi. 1888, p. 135. 



4 Busk, "Challenger" Reports, Parts 30 and 50. 



5 Hincks, Brit. Marine Polyzoa, Introduction, p. exxxv. 



