309 



REPRODUCTIVE ORGANS 



The sexes of the lancelet are separate, but show no differences 

 save in the nature of the gonads. These are cubical bodies, 

 twenty-six on each side, situated in the wall of the atrium, into 

 which they shed their products by rupture of their walls. Each 

 corresponds to one of the myomeres and consists of a closed 

 coelomic sac, whose cavity is known as the gonocoele and on 

 whose wall the gametes arise, though they are actually derived, 

 by a rather complicated process, from the epithelium of the embry- 

 onic coelom of the myomere behind that in which they lie. The 

 egg is minute, but contains yolk granules. The gametes are shed 

 in spring, and are carried out by the current through the atriopore 

 and fertilisation takes place in the water. The further develop- 

 ment is described in Chapter 28. 



CHORDATA 



The lancelet is a member of the phylum Chordata, and possesses 

 most of the features of the group in an almost diagrammatic 

 form, uncomplicated by the specialisations and comphcations 

 of the higher types. The chordates resemble the annelids in being 

 segmented coelomate Metazoa, but they differ from all other 

 phyla in possessing the following four structures : a dorsal noto- 

 chord ; a nerve cord which is hollow, single and dorsal ; visceral or 

 gill clefts opening from the pharynx to the exterior ; and a post- 

 anal tail containing no viscera ; further, the blood flows forward 

 in the ventral vessel. All these we have seen in the lancelet, 

 but in some of the other primitive chordates one or other of 

 the characteristics may be missing, and in the higher forms they 

 tend to be lost or obscured in the adult. Apart from possession 

 of some of the fundamental characters of the phylum, the 

 primitive groups have little resemblance to each other, and are 

 therefore most conveniently arranged as subphyla. Such a division 

 makes four sections of the chordates, thus : 



SUBPHYLUM I— HEMICHORDATA 



These are well described by their name, for they are only half 

 chordates, or less. One class, the Enteropneusta, represented by 

 Balanoglossiis, a marine worm-hke animal, has gill sUts, a partially 



