MICROSCOPICAL STUDIES. 49 



The anatomy and description of few animals are better known, 

 hence I have little or no excuse for occupying space with details here. 

 Any zoological text-book will provide such, as for example Marshall 

 and Hurst's " Practical Zoology" which is specially commendable, 

 from its clearness and accuracy. I will therefore now give what 

 is intended as merely a running commentary upon the figures as 

 drawn on Plate IV. 



External Characters. — In life, the Lancelet is a small, nearly 

 transparent fish-like animal, pointed at either end — of some two 

 to three inches in length. No scales are present, the body being 

 clothed in a thin transparent cuticle. Along the entire dorsal region, 

 and along the ventral from atrial pore to the end of the body, the 

 skin is raised into a fold or ridge best marked both dorsally and 

 ventrally in the part of the body posterior to the anus, and which we 

 may term the tail. The dorsal ridge of skin is strengthened, except 

 at the ends, by a row of cube-like compartments forming a supporting 

 skeleton, answering to the fin rays of true fishes. The ventral ridge 

 is strengthened by a similar skeleton between atrial pore and anus, 

 and this part is the ventral fin (vf.) ; the fold running along the 

 dorsal and ventral sides of the tail being the caudal fin, while the 

 dorsal fold anterior to the tail constitutes the dorsal fin (df.) 



Alimentary Canal. — The mouth is a large oval aperture 

 placed anteriorly on the ventral aspect, and bordered on either 

 side by a line of tentacles (bt). The mouth leads into an elongated 

 dilated chamber, the pharynx, which functions also as the organ of 

 respiration (pli). Its walls are supported on either side by a frame- 

 work of cartilaginous arches set obliquely, the gill arches (ga), along 

 which course the branchial blood-vessels. Between every two arches 

 is a long narrow perforation, the gill cleft. Through these openings, 

 the water, brought in through the mouth by the action of the cilia 

 that line the phar} T nx, is directed into a cavity all but surrounding 

 the pharyngeal tube. This, the Atrial Chamber, corresponds closely 

 to that of similar name in the Ascidians, and indeed, the respiratory 

 plan is altogether exactly conformable in the two animals. The 

 water passing into the atrial cavity from the pharynx is conveyed 

 out by an opening, the atrial pore (<xp), situated at the front end 

 of the ventral fin. As in the Ascidians, an endostyle is present 

 in the ventral wall of the pharynx. It is however flattened in the 

 anterior part, but posteriorly it has the deeply grooved form charac- 

 teristic of Ascidians. The epibranchial groove found in Ascidians 

 is also well marked as a dorsal counterpart of the endostyle. 

 The pharynx leads into a straight intestine ending in an anal 

 opening (a) placed on the left side of the ventral fin. 



