704 THE ANIMAL KINGDOM. 



vessels by valved inlets. It divides ; the branches surround the oeso- 

 phagus, and unite to form a second, the longitudinal ventral vessel, 

 which runs towards the base of the body in the left ventral chamber of 

 the coelome giving off many lateral caecal branches. There is a vascular 

 sinus round the stomach. The vessels are all contractile and contain 

 nucleated corpuscles tinted with haemoglobin (or haemerythrin ?). A 

 fatty tissue surrounds the lateral caecal vessels above named. In it are 

 developed, but on opposite sides, the spermatozoa and ova. The latter are 

 impregnated in the coelome, and pass out thence by two ciliated tubes or 

 nephridia, which open internally into the single one of the three posterior 

 chambers of the coelome, and externally near the anus. The ova adhere 

 to the tentacles. Segmentation is total, but unequal. The larger cells 

 are invaginated, and the blastopore persists as the mouth. The larva is 

 known as Actinotrocha. It has a large praeoral hood-like lobe with strongly 

 ciliated edges : an imperfect postoral circlet of ciliated larval tentacles : and 

 a ring of large cilia round the anus which is a proctodaeum. The body is 

 also ciliated. There is a supra-oesophageal ganglion in the praeoral lobe, 

 a sense organ in some species, and in one, four eye-spots. At the meta- 

 morphosis the praeoral lobe, ganglion, and larval tentacles, are swallowed 

 and digested after disruption along a line of vacuolated ectoderm cells 

 (Caldwell). The body, at first developed as a ventral invagination, is 

 suddenly evaginated, and a new set of tentacles is formed. 



Caldwell, P. R. S. xxxiv. 1883; Ray Lankester, Encyclopaedia Britannica (ed. 

 ix), xix. p. 433. 



For Actinotrocha, see also Wilson, Q. J. M. xxi. 1881. 



CLASS POLYZOA. 



(Bryozoa). 



Coelomate Metazoa, small in size, and, with one exception, forming 

 colonies, or zoaria, which are, as a rule, fixed. There is a cuticle, secreted by 

 the ectoderm, usually thickened and rigid on the posterior part of the body, but 

 flexible on the anterior part. The ectoderm is generally unilaminar. The 

 nervous ganglion is single. The mouth and anus are approximated : both of 

 them may lie within the lophophoral area (Entoprocta), or only the moiith 

 does so (Ectoprocta). The digestive tract is U-shaped, its flexure ventral in 

 Entoprocta, and, perhaps so, in Ectoprocta : it is always partially ciliated. 

 The larva is free- swimming. Gemmation is universal. 



There are two sub-classes, the Entoprocta and Ectoprocta, which are 

 better considered apart. 



Entoprocta. Two well-known genera, both marine, are comprised in 



