412 CEPHALOCHORDA. 



and other small organisms, which are sucked into the mouth. 

 At times, especially in the evening, the adults start up and 

 swim about, but they are never so active as the larvae. The 

 early embryo is pelagic. It is of interest to note that along 

 with lancelets, specimens of the Annelid Ophelia are often 

 obtained ; they closely resemble lancelets, not only in shape 

 and size, but also in the way they burrow and swim. 



Form. — The body, between \\ and 2 in. in length, is 

 pointed at both ends, as the names suggest. The living 

 animal is translucent, with a faint flesh colour, and is much 

 plumper than a spirit specimen. The muscles are arranged 

 in sixty-two segments or myotomes. There are three un- 

 paired apertures — (a) the median, ventral, pre-oral hood 

 over-arching the true mouth, and fringed with tentacle-like 



G. 



Fig. 177. — Lateral view of Amihioxus. — After Ray Lankester. 



The notochord runs from tip to tip. 



t., Tentacular cirri; G., reproductive organs; a.p., atriopore ; 



a., position of anus ; 40 and 62, indicate number of myotomes. 



cirri ; (6) the atriopore in myotome thirty-six, giving exit to 

 the water which enters by the mouth ; (c) the anus, ventral 

 and slightly to the left, behind the atriopore, but some 

 distance from the posterior end of the body. Along the 

 back there is a median fin, which is continued around the 

 tail, and along the ventral surface as far as the atriopore. 

 In front of this region the ventral surface is flattened, and 

 fringed on either side by a slight fin-like " metapleural " 

 fold. These folds are continuations downwards of the walls 

 of the atrial or branchial chamber, which extends from 

 behind the mouth to the atriopore, and into which the gill- 

 slits of the pharynx open in the adult. 



Skin. — The epidermis consists of a single layer of cylin- 

 drical cells. Some of them project slightly from the surface, 

 and are connected at the base with nerve fibres. These are 

 sensory cells, and may be compared to the cells of the 



