394 ZOOLOGY FOR MEDICAL STUDENTS chap. 



" stomach " grows out to form a pocket which extends forwards for 

 some distance on the right side of the pharynx. This pocket is of great 

 morphological interest for it represents the Hver of the typical verte- 

 brates which in Amphioxus retains through life the simple pocket-like 

 form that in other vertebrates is found only in the early stages of its 

 development. 



We have seen how in the Teleostomes and Lung-fish the external 

 openings of the gill-clefts become covered in by the opercular flap so 

 that they no longer open on the outer surface of the body but open into 

 the opercular cavity. In analogous fashion the numerous gill-slits of 

 Amphioxus open into an extensive cavity called the atrium which opens 

 to the exterior by a mid-ventral opening — the atriopore (Fig. 172, at). 



The coelome is in the pharyngeal region reduced to small remnants, 

 its place being in great part occupied by the atrium, but in the intestinal 

 region it is seen surrounding the alimentary canal in normal fashion. 

 The nephridia are short squat organs opening into the atrium near the 

 dorsal ends of the gill-slits. They are unique amongst vertebrate 

 nephridia in two respects : (i) that at their coelomic ends they are 

 provided not with open nephrostomes but with numerous solenocytes 

 (Fig. 75, E, p. 162), and (2) that they do not open into a common 

 longitudinal duct. 



Combined with this highly abnormal condition of the nephridia 

 themselves we find in Amphioxus complete absence of the genital ducts 

 seen in normal vertebrates. The testes, or ovaries, form segmentally 

 arranged masses which as they increase in size bulge into and finally 

 burst into the atrial cavity, the gametes passing to the exterior through 

 the atriopore. Some of the most important features of Amphioxus 

 have to do with its embryology and these will be referred to later in 

 Chapter XIV. 



The blood of Amphioxus is colourless : the general arrangement of 

 the vessels is on the lines normal to the vertebrate. The chief peculiarity 

 to be noted is that there is no special concentration of the contractility 

 of the sub-pharyngeal vessel in the region of the normal heart but that 

 there are such concentrations at the ventral end of each aortic arch — 

 each arch having developed a special little bulb-like heart of its own. 



A notochord is present, enclosed in a thin sheath, and extending 

 from end to end of the body. The peculiarity that it extends to the tip 

 of the head, in which Amphioxus is unique, is expressed in the group- 

 name Cephalochorda. The connective' tissue of the body — very sparse 

 — acts as a skeleton in places, forming tough envelopes round the noto- 

 chord and central nervous system, and forming septa between the 



