236 DESCRIPTIONS OF PREPARATIONS. 



proximal portion is retained in a folded condition by certain ligaments and muscles. 

 The summit of the fold forms a circular 'diaphragm' (d.\ which contains a 

 muscular sphincter, and closes over the tentacle-sheath (/.) when it is invaginated. 

 The walls of the tentacle-sheath are homogeneous and contain nuclei, and are 

 probably covered, as in other Polyzoa, by a delicate cuticle. It is crowned by a 

 circlet of hollow, ciliated tentacles (T.\ usually eighteen in number. The mouth 

 (m.) lies in the centre of the circlet, and the cilia on the oral surfaces of the 

 tentacles which work towards the mouth are longer than those on their aboral 

 surfaces. The digestive tract consists of a more or less cup-shaped pharynx or 

 oesophagus (<?.) separated by a constriction from the stomach, which is divisible 

 into a cardiac tubular portion (r.), a caecal sac (st.) and a pyloric region (P.), which 

 is bent upwards on the distal or anal surface of the cardia. It ends with a short 

 rectum (.). The true anus (a.) is situated at the bottom of a tubular depression 

 of the tentacle-sheath. The walls of the digestive tract are composed of a homo- 

 geneous lamella, lined internally by an epithelium, ciliated in the first part of the 

 cardia, and the pylorus, while the non-ciliated cells of the stomach contain a 

 brownish pigment ; and coated externally by a layer of nucleated protoplasm, which 

 passes at the base of the stomach into the fusiform cells of the funiculus (/'.) with 

 its various branches. These are connected to the funiculi laterales and the endo- 

 cyst of the proximal wall of the zooecium. The nerve-ganglion (n.} is placed 

 between the oesophagus and anus. According to Hincks (British Marine Polyzoa, 

 i. Introduction, p. Ixxxix), a ciliated intertentacular tube is often present in this 

 species. Spermatozoa have been seen to pass outwards through it, but it is also 

 present in the female zooid. 



The muscles present in a zooecium are (i) parietal muscles (pm.) formed by 

 bundles of two to five fibres, attached to the basal angle of the sides at one end 

 and to the upper surface at the other. This upper surface they depress, (ii) A 

 pair of opercular muscles (op.m.}. (iii) A retractor muscle (rm.), which invaginates 

 the tentacle-sheath and is attached at one end to the abanal side of the oesopha- 

 gus, at the other to the proximal wall of the zooecium. (iv) A pair of parieto- 

 vaginal muscles (J>v.m.} attached to the base of the fold surrounding the tentacle- 

 sheath, and situated close to the opercular muscles on the anal side of the digestive 

 tract, (v) The muscles of the tentacle-sheath. These are arranged in two layers 

 a circular, which forms a sphincter near the base of the circle of tentacles ; a longi- 

 tudinal, the fibres of which are collected towards the base of the sheath into four 

 bundles two on the abanal, two on the anal side. The two first form the superior 

 parieto-vaginal ligaments (/#./.), and are attached to the upper zooecial wall'; the 

 two latter form the inferior parieto-vaginal ligaments (/#./'.), and are attached to 

 the inferior zooecial walls close to the angles of the distal end. 



The parent zooecium of the colony has a rounded outline, and gives off 

 numerous buds. In the colony, when once established, new zooecia are formed as 

 buds in the same linear series as the old zooecia. A bud of unusual width may 

 divide longitudinally, thus originating two parallel series in the place of one. The 

 endocyst of the growing bud is composed of two distinct cell-layers an outer set 

 of columnar cells, an inner set of fusiform cells. The latter persist in the funiculi 

 laterales, and other cords of cells, but are indistinct in other parts of the zooecium. 

 In the fresh-water Polyzoa there is an inner layer of ciliated cells. The new 





