64 



ZOOLOGY 



SECT. 



protovertebi'ffi in the form of pouches, which gTadnally assume 

 their ])ermanent form. The development of the nephridia has not 

 been fully worked out ; but what is known regarding it su})ports 

 the conclusion that these organs, while representing the nephridia 

 of the Annulata, do not correspond to the excretory organs of the 

 Craniata (see Section II.). 



Distribution. — The Branchiostomida' are very widely dis- 

 tributed in tropical and warm-temperate seas. Amj^hioxidcs has 

 only been obtained with the tow-net and is, seemingly, of 

 permanently pelagic habit. It differs from Amphioxus in 

 the absence of an atrial cavity, the branchial slits opening 



Fig. 756. — Amphioxus lanceolatus. Diagrammatic transverse sections of Uirco larvae 

 to show the development of the atrium, ao. aorta ; c, dermis ; il, intestine ; /. fascia 

 (layer of connective-tissue on inner surface of myomere) ; .?7(, cavity for dorsal fin -ray ; m. 

 myomere ; n, neuron ; ji, atriiim ; .';/', metaplcural folds ; si, sub-intestinal vein ; si; sheath 

 of notochord and neuron ; si. sub-atrial ridge ; sp, coelomc. (From Korschelt and Heider, 

 after Lankester and Willcy.) 



directly on the exterior. No sexually-mature specimens have yet 

 been found. 



Distinctive Characters. — The Acrania may be defined as 

 Euchorda in which the notochord extends to the anterior end of 

 the snout, in advance of the central nervous system. There is no 

 skull, and no trace of limbs. The ectoderm consists of a single 

 layer of cells which may be ciliated. The pharynx is of immense 

 size, perforated by very numerous gill-slits, and surrounded by an 

 atrium. The liver is a hollow pouch of the intestine. There is 

 no heart, and the blood is colourless. The nephridia remain dis- 

 tinct and open into the atrium. The brain is very imperfectly 

 differentiated ; there are only two pairs of cerebral nerves ; and the 



