1884.] Balanoglossus Kowalevsldi (Agassiz), fyc, 25 



region, which may thus be distinguished from the pharyngeal region 

 in front and the intestinal tract behind. In the walls of the former 

 the gill-slits are placed. They increase in number from before back- 

 wards, throughout larval and adult life. Their openings are at first 

 circular, but afterwards become horse-shoe shaped, owing to the 

 downward growth of a process from the dorsal margin of their open- 

 ings. This process (the " valve " of Spengel) appears about a week 

 after the first appearance of each gill-slit. By the continual dorso- 

 ventral elongation of the gill-slits, together with an antero-posterior 

 compression, their openings become [J -shaped. 



The digestive region is yellow-brown in colour. Its walls are 

 thrown into a spiral fold, giving it a sacculated appearance when seen 

 in transparent specimens. The intestinal region is straight, and long 

 cilia may be seen working in its interior, causing an outward current. 

 The anus is now dorsal, and can only be closed by an indrawing of the 

 intestine. 



(8.) A large vesicle may be seen pulsating in the base of the pro- 

 boscis, as described in Tornaria by previous observers. In the dorsal 

 middle line is a longitudinal vessel, which contracts peristaltically, 

 like that of B. minutus (Spengel). These contractions appear to 

 have no constant direction. The vascular movements are not corre- 

 lated to movements of the body. No contractions were seen in the 

 ventral vessel, though their absence was not satisfactorily shown. No 

 further points of importance can be seen in an examination of the 

 larvae as whole objects. 



It has been stated by Spengel with regard to B. minutus, that water 

 is taken into the body cavity of the proboscis at the proboscis-pore, 

 and into that of the collar by the two ciliated funnels which open 

 into it. My own observations do not confirm either of these state- 

 ments ; on the contrary, particles of Indian ink or carmine held in 

 suspension in the water cannot be found to enter into either of these 

 cavities, while if placed artificially in them are driven out at all three 

 of these points. On the other hand the particles are carried into the 

 alimentary canal by the ciliary currents, and are more or less expelled 

 thence at the gill-slits. Spengel's statement of the absence of the 

 pore described by Kowalevsky and Agassiz, at the apex of the pro- 

 boscis, is true for all the species which I have examined. 



Internal Structure. 



Alimentary Canal. — As mentioned above, the mouth is anteriorly 

 directed. The gill-clefts arise as saccular paired evaginations from 

 the pharynx which come into contact with the skin. The two layers 

 coalesce, become very thin, and then break down, forming openings 

 into the alimentary canal, which are at first circular. In young speci- 



