ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 43 



raphe into the branchial cavity, but ascends towards the mantle to the 

 pericoronal groove, where it forms a curtain which collects the nutrient 

 particles and directs them towards the oesophagus. This arrangement 

 is a proof of the homology of the endostyle with the thyroid gland of 

 Cyclostomata and Selachians. The mediodorsal or interserial " lan- 

 guettes " are simple expansions of the interserial plates, and their function 

 is to direct towards the entrance of the oesophagus the cord of mucus 

 which is formed at the level of the pericoronal circle. They are not 

 mobile, and act only by their vibratile cilia. The posterior raphe or 

 retropharyngeal band is formed by a projecting crest which lies in the 

 prolongation of the right lip of the endostyle, and extends from the 

 posterior cul-de-sac of the hypobranchial groove as far as the oesophagus. 

 Its left surface only is invested by a vibratile epithelium, which is 

 directly continuous with that of the two lips of the endostyle. 



The peribranchial cavity, which is the subject of the fifth chapter, is 

 made up of a large undivided region situated in the mediodorsal line of 

 the Ascidian, the cloacal cavity, and from thirteen to sixteen caecal 

 prolongations, which surround the branchia except in the medioventral 

 line, for they do not extend underneath the hypobranchial groove. The 

 investing epithelium is flat, and identical with that of the epidermis and 

 of the gill. The cloaca receives the blood which comes from the gill as 

 well as the excreta and genital products of the organism; the anus opens 

 by a wide space in its lower part, and the genital ducts open just 

 opposite the anus. The cloacal siphon, which has the same structure 

 as the buccal, is placed in the upper part of the cloaca ; it has, like it, 

 transverse and longitudinal muscles, but the latter are not found in a 

 dorsal appendage of the siphon, where they are represented by fibres 

 given off from the transverse muscles. This fact is of some morpho- 

 logical importance, for it tends to prove that the muscles which encircle 

 the entire body of Doliolum, and which also give off prolongations to 

 the anal appendage of these animals, are, in Ascidians, homologous not 

 with the circular muscles of the body, but with the transverse muscles 

 of the siphon. When ova are produced, the hinder part of the cloaca 

 dilates considerably, so as to form an incubating pouch in which the 

 eggs are developed. The observations of the author tend to confirm 

 the statement of MM. Van Beneden and Julin as to the origin of the 

 layers of the peribranchial cavity. In Fragaroides, as in simple 

 Ascidians, the parietal layer of the peribranchial cavity has an ecto- 

 dermic, and the visceral layer an endodermic origin. 



The digestive tube is described in the next chapter. It is placed 

 altogether behind the branchial cavity, and is composed of oesophagus, 

 stomach, and intestine; the last may be subdivided into duodenum, 

 chylific ventricle, and rectum. The orifice of the oesophagus is elongated 

 in such a way as to advance towards the posterior cul-de-sac of the 

 endostyle ; the vibratile cilia of its epithelium are prolonged into the 

 interior of the cell as far as the deep granular mass, and the protoplasm 

 becomes thickened around the base of each cilium. The cilia and their 

 prolongations are broken up into small dots, which are set in regular 

 lines transversely as well as longitudinally. The stomach is cylindrical 

 in form, and is marked by eighteen to twenty grooves, the centre of 

 which alone communicates with the cavity of the stomach ; this is 

 owing to the fact that both the oesophagus and the intestine project into 

 the interior or the gastric cavity and foi'm a kind of valve. These 



