POLYZOA. 241 



lipora and Idmonea are common in the shallow waters north of Cape Cod, one species 

 of the former genus extending to the south of that barrier. 



Stjb-Oedbe II. — Ctenostomata. 



Here the cell is closed, after the retraction of the polypide, by processes of the 

 tentacular sheath, or by bristle-like projections. Two families, represented by several 

 species, occur on our coasts. In the AlcyonidiidjB the colony forms a fleshy or mem- 

 braneous mass very irregular in form. Our most common species is Alcyonidium 

 hispidum, and scarcely less frequent is the closely related A. Mrsutum. These forms 

 are found most abundant surrounding the stems of tlie rock-weed {Fucus), between 

 tide-marks. The former is thicker, and may be readily recognized by the slender 

 reddish bristles which surround the mouth of the cells. In the second species each 

 cell forms a small soft papilla, from the centre of which the polypide protrudes itself. 

 A. ramosum is a large branching species, which not infrequently forms colonies over 

 a foot in length, the branches sometimes being nearly half an inch in diameter. In 

 the Vescictjlaeid^ the general form of the colony is a creeping or upright branch- 

 ing mass, from which the cells arise as free sheaths. In Vescicidaria these sheaths are 

 sessile upon the stock, while in Farella they are seated upon short peduncles. Several 

 species of the former are common upon our coasts, while Farella familiaris, which 

 extends from Long Island Sound to Europe, is found on rocks and sea^weed. " When 

 it surrounds the stems of small algiB, the whitish pedicels project outwards, in all di- 

 rections, and thus produce the appearance of a delicate chenille cord." The members 

 of the family PALtroicELLiD^ are inhabitants of fresh water. 



Sub-Order III. — Chilostomata. 



The Chilostomata are characterized by having the mouth of the horny or calcareous 

 cell capable of being closed by a lid, while the oral area is usually membranous, rather 

 than horny. It is in this group alone that we meet with the vibracula and avicularia, 

 which we have already described, but their presence is not universal. It is divided 

 into four super-families, the lowest being the CELLULARINA. Here the horny or 

 slightly calcareous cells are tubular, funnel-shaped, the lower attached 

 extremity being tubular or conical. In ^ta anguinea, which may be 

 taken as the type of the family ^tid^, the tubular cells, with the mouths 

 at the apex, arise from a creeping root-stalk, while in Eucrate chelata, 

 representing the Euceatid^, the mouth of the cell is one side of the 

 extremity, and the cells are arranged in a single row. In Oemelluriu 

 loricata, a large form common in shallow water north of Cape Cod, we 

 have a close similarity to the last species, except that the cells are 

 arranged in pairs, back to back. In the Eucratidae no vibracula or 

 avicularia are present, while in the Cellulaeid^ these structures are fig. 240.— Gem- 

 usually found. The colony branches dichotomously, and the cells ai-e ' """' 

 arranged in two or more rows. It is represented in our waters by species of Cellu- 

 laria and Caberea. 



In the BicELLAEiD^ the cells are conical or quadrangular, and the large, laterally 

 placed mouth is placed near the median axis. Our most prominent species of this 

 family belong to the genus Bugula. Here the branches are arranged in a spiral, giving 



VOL. I. — 16 



