Introduction to A nimal Morphology. 229 



growth of the embryo in Rhabdopleura there are two fleshy 

 side lobes attached along the dorsal edge, which do not in- 

 crease as growth progresses, and are supposed by Alimati to 

 be the homologues of the mantle lobes of lamellibranchiate 

 INIolIuscs. Rounded gcn7i capsjiles are found inside polypide 

 cells (zooecia), of which the original inhabitant has died. 

 These are formed from protoplasmic masses below the fundus 

 of the stomach, and originate new inhabitants for the deserted 

 house. A second rarer way for new inmates to arise is by 

 budding from the endocyst. 



About 600 species exist, divided into two orders : — 



I. Gymnolaemata {Allman) — with no epistome, a funnel- 

 shaped mouth, and the 9-16 tentacles on a circular lopho- 

 phore ; they never develop statoblasts. This includes six sub- 

 orders : — 



Sub-order i. Cyclostomata — calcareous, porous, each per- 

 sona retractile into its cell ; mouth terminal, wide ; avicularia 

 and vibracula none ; cell-mouth with no crown of bristles ; 

 they are mostly fossil (Mesozoic) ; some have a jointed, 

 colonial axis (Crisiadae) ; others are unjointed, and with 

 opercula to the cells (Myriozoidae) ; the cells may have thin 

 mouths, separate, protruding (Tubulata), or united in project- 

 ing bundles (Fasciculata). 



Sub-order 2. Ctenostomata — retractile, with terminal, wide 

 mouth, and no avicularia nor vibracula ; the oval or cylindrical 

 cells not pillared, but though separate, arising from a common 

 stalk ; cell mouth with a setose margin for its closure. The 

 colony may be horny and crust-like, freshwater, each cell joined 

 to 4-6 neighbour cells (Hislopiadae, a passage form), or they 

 may be marine, spongy, massive, or crust-like ; the imbedded 

 cells with contractile mouths ^(Alcyonidiadae), or plant-like 

 branched marine forms, upright or creeping, with projecting 

 cells. A gizzard is always present in Vesiculariadae. 



Sub-order 3. Chilotomata — lophophore and sheath pro- 

 trusible ; cell mouth narrower than the cell, placed at the 

 front near the terminal pole ; mouth often with a horny 

 operculum, or a sphincter ; avicularia and vibracula often 

 present, rarely very large and ^numerous, used as creeping 

 organs (Selenariadoe) ; the colony in the other families is 



