ANATOMY AND DEVELOPMENT. 195 



In both cases the pharynx is perforated by a great num- 

 ber of gill-apertures (gill-slits, stigmata), converting it into 

 a branchial sac and opening into an atrial or peribrancliial 

 cavity instead of directly to the exterior. At the base of 

 the pharynx there is a longitudinal gland consisting of a 

 groove open throughout its whole length towards the cavity 

 of the pharynx, and known as the endostyle, whose histo- 

 logical character is closely similar in the two cases. From 

 the anterior extremity of the endostyle a ciliated band of 

 columnar cells passes round the wall of the pharynx on 

 each side, in front of the gill-openings, and abuts on the dor- 

 sal border of the pharynx, along which it is continued back- 

 wards in connexion with the dorsal lamina in the one case 

 and the hyperpharyngeal groove in the other. This band 

 forms a circlet round the pharynx behind the velum, and is 

 the peripJiaryngcal band* We shall find also that the 

 Ascidian hypophysis is essentially homologous with the 

 olfactory pit of Amphioxus. 



In the Ascidians there are sphincter muscles round the 

 buccal and atrial siphons, and inside the former, in front of 

 the peripharyngeal band (pericoronal groove), there is a 

 circlet of tentacles corresponding perhaps to the velar 

 tentacles of Amphioxus. (Cf. Fig. 94, /.) 



The differences between the structure of an adult Ascid- 

 ian and of Amphioxus may appear to outweigh the resem- 

 blances, but it must be remembered that they are all 

 correlated with and accessory to the one great difference 

 in the mode of existence of the respective types. 



An Ascidian is sessile ; Amphioxus is free. The former, 

 as it were, builds its house upon a rock and is immovable ; 

 the latter lives in the shifting sands, and is capable of 

 extremely active locomotion. 



* As mentioned above, this band is usually grooved in the Ascidians. 



