64 PHYLUM TUNICATA (UROCHORDA). 



vesicle appears to be formed by the ganglion. There are no 

 visual organs. The notochord (h) is confined to the tail. It 

 consists of an axial hyaline substance of cartilaginous consistency, 

 surrounded by a protoplasmic nucleated membrane. The nuclei 

 project on the inner side into the hyaline axial substance. The 

 musculature of the tail is well developed. It consists of two 

 broad bands of muscular substance, one on each side of the noto- 

 chord. Each band consists of an outer protoplasmic layer con- 

 taining the nuclei and an inner layer of striated contractile 

 substance. The nuclei are ten in number in each band, arranged 

 in a row at regular intervals, thus indicating that the muscular 

 bands are composed of ten cells. The limits of these cells can- 

 not be made out in the adult ; though it is said that after certain 

 treatment indications of them may be seen as transverse lines 

 through the muscle substance between the nuclei. (Fig. 55, k). 

 It has been suggested that these lines represent the limit of 

 segments and that the tail is segmented. This however is very 

 doubtful. There is no correspondence between the caudal 

 ganglia and the supposed muscle segments. 



The pericardium is a simple epithelial sac placed on the ventral 

 side of the stomach. Its dorsal wall is contractile and slightly 

 invaginated into its cavity. A concave contractile lamella 

 (Fig. 55) bounding a blood sinus, the dorsal wall of which is 

 formed by the stomach, is thus established. This concave 

 contractile membrane constitutes the heart. When it contracts 

 it propels the blood from the super jacent blood sinus into the 

 blood channels generally and so acts as the central organ of the 

 circulation. It is said to reverse the direction of its action as 

 in other Tunicates, but this is doubtful. 



The wall of the pericardium is a simple protoplasmic membrane con- 

 taining nuclei. Dorsally it contains in its outer layer striated contractile 

 fibres. It differs from the heart of other Tunicates merely in the small 

 extent to which the dorsal wall of the pericardium is invaginated. 



The gonads are contained at the posterior end of the body 

 (Fig. 55, /, g). Excepting Oikopleura dioica all members of the 

 group are hermaphrodite, the male organs maturing first. There 

 is a single or double ovary contained between two testes. There 

 is no oviduct. The eggs when ripe dehisce into the vascular 

 cavity from which they escape by dehiscence of the body- wall, 



