Invertebrate Gallery of the Indian Museum, 51 



shallow water, and Terebratula from the depths of the 

 Laccadive Sea ; but Australian, Mediterranean, and Atlan- 

 tic forms are well represented. 



3. POLYZOA. 



[®e0hm ®JaU-i:a0C0 44-45]. 



The Polyzoa are small animals which bud asexually, 

 like many of the Hydrozoa and Actinozoa, to form branch- 

 ing or encrusting shrub-like or moss-like colonies. Some 

 live in freshwater, but the majority are marine. The 

 colonies are supported and protected by a horny (chitinous) 

 or calcareous exoskeleton formed by the hardening of 

 the outer layer of the ectoderm of their units or zooids. 

 It is these horny and seaweed-like, or calcareous and 

 coral-like, exoskeletons, denuded of their occupants, which 

 form the majority of museum specimens of Polyzoa. 



A good example of a Polyzoon colony is the specimen of 

 Retepora in Case 45. This is seen to be an encrusting 

 branching lichen-like mass, the thin leaves of which are 

 formed of carbonate of lime and have the texture of a very 

 fine lace with very regular meshes. The meshes of the 

 lace-work are the openings of chambers which, during 

 life, were occupied by the Polyzoa zooids. The colony, 

 in short, consists of myriads of these little chambers, all 

 shut off from one another, in each of which dwelt a small 

 animal, or zooid, of the shape and structure shown in the 

 enlarged diagrammatic drawing alongside of the specimen ; 

 and each little chamber was formed simply by the calcifica- 

 tion of the cuticle of its occupant. During life the zooids 

 ordinarily extrude their head and tentacles from the open- 

 ings of the chambers, as shown in fig. i, but they are able, 

 when alarmed, to completely withdraw within the cham- 

 bers, and sometimes to close the orifice, as shown in fig. 2. 



In many of the Polyzoa colonies a division of physiolo- 

 gical labour occurs, with the result that some of the zooids 

 lose all resemblance to the typical form. Certain of them 

 become transformed into structures resembling a bird's 

 head, and these, which are known as avicularia and are 



D 2 



