956 SUMMARY OF CURRENT RESEARCHES RELATING TO 



branchial cavity. These intcrscrial plates almost divide- tho branchial 

 cavity into a 6erics of secondary cavities. They arc not even inter- 

 rupted on the dorsal surface, bnt aro directly continuous with tho 

 median dorsal langucts, which are to bo regarded as appendages of 

 the " intcrscrial plates." (r) Inside each of the transverse sinuses 

 and intcrscrial plates a pair of muscles extend, side by side, through- 

 out their entire length. These extend all round the branchiae, except, 

 of course, at tho level of tho endostyle, where there are no transverse 

 sinuses. They aro united by numerous anastomoses to tho longi- 

 tudinal muscles of the internal tunic. 



Tho retropharyngeal tract docs not form a gutter ; for a largo 

 portion of its course it appears merely as a ciliated ridge projecting 

 into tho branchial cavity. Only the right margin of the furrow is 

 developed ; the ridge is, on the one side, continuous with the two lips 

 of the endostyle, and loses itself on tho other side in the oesophagus. 

 The cells round the stigmata are elongated in the direction of the 

 length of the latter. They are arranged in transverse rows of six 

 colls. Each bears a projecting ridge with 11-15 cilia. 



II. Nervous system. — The ganglion consists of a fibrillar mass 

 with several series of irregularly disposed, peripheral ganglion cells. 

 The visceral cord has a similar structure. It is prolonged between 

 tho branchial and cloacal epithelium towards the visceral mass. It is 

 surrounded by large blood-spaces, and accompanied, throughout its 

 entire length, by lateral muscle-bundles. The hypoganglionic gland 

 consists of a mass of cells, markedly degenerating towards the centre. 

 The discharge is got rid of by the aid of the vibratile organ, and 

 passes off between the tentacular crown and the external lip of the 

 pericoronal furrow. Posteriorly, the gland is continued into a canal, 

 which loses itself in'connection with the visceral nerve-cord. At an 

 early stage the excretory duct of the as yet unformed gland is con- 

 tinuous with the lumen of the visceral cord, then also a tube. 



III. Muscular system. — Apart from the muscles round the two 

 siphons, only longitudinal muscles occur in tho tunic. Tbese are all 

 lateral, none are in reality median. Each bundle ends near the 

 extremity of the post-abdomen in a knob-like projection. The 

 muscles consist of homogeneous, unstriated fibrils surrounded by a 

 fine sarcolemma, and including between them masses of nucleated 

 protoplasm. 



Polyclinse.* — M. Lahille considers that tho great polymorphism 

 of the genus Sidnyum has led various authors to rename species 

 of this genus as belonging to other genera : e. g. Circinalium con- 

 crescens is really S. turbinatum. He divides the Polyclinae into two 

 families : the Polyclinidae, and the Aplididae, the characteristics of 

 which are given. The first family are remarkable amongst other 

 points for the " reproductive appendix," the post-abdomen of other 

 authors. It contains a flattened cavity, the " endoderniic tube," 

 bifurcated at each end. The cavity is originally a prolongation of the 

 branchial chamber, and is placed ventrally. This endoderniic tube 



* Comptes Rendus, ciii. (1886) pp. 485-7. 



