LATEK LARVAL STAGES. 561 



WlJHE, like th<' larval mouth, an organ belonging to the left side of the body, 

 in spite of its apparently symmetrical position. 



It is difficult, from Wii.lhy's description, to gain a clear idea of the sh if tings 

 which occur in the oral region. According to this author, the longitudinal axis 

 of the slit-like larval mouth rotates about a vertical axis through an angle of 

 90 c . Originally it lies parallel to the axis of the body, but is at right angles 

 to it later. Consequently, the anterior part of the larval mouth sinks in and 

 passes to the right side to give rise to the right half of the velum, while the 

 left half of that organ is derived from the posterior corner of the mouth. 

 Similar shifting of the labial folds takes place. 



van Wijhe maintains that the mouth of Amphioxtts is not homologous 

 with that of the Craniata. He also doubts the homology of the velum of 

 Ampkioxus with that of the (Jyclostomi. According to him the mouth of 

 Amphioxus is a gill-cleft lying on the left, and the club-shaped gland is the 

 corresponding organ on the right side of the body, van Wijhe homologises 

 it with the left spiracle of the Selachians aud with the left gill-cleft of 

 Appendicularia (.'i. 



* 



In the later larval stages, when eight secondary gill-clefts have 

 already developed and the tongue-bars have begun to form, the 

 club-shaped gland degenerates, and finally, as it appears, reaches 

 the interior of the alimentary canal where it is perhaps absorbed. 

 The aperture of the gland into the intestine which was mentioned 

 above (p. 550) seems to withstand disintegration longer than any 

 other part of the organ (Willey). 



In these later stages an organ is found which was discovered by 

 Hatschek (No. 5) and was figured and described both by him and 

 by Kay Lankester and Willey (Nos. 12 and 23)' as the kidney 

 {nephridium, Fig. 290, x). This is developed only on the left side in 

 the form of a mesodermal ciliated funnel and canal lying in front of 

 the mouth in the region of the first metamere (between the pre-oral 

 pit and the mouth). It lengthens posteriorly later and is found, in 

 the adult, as a strand extending on the left side beneath the chorda 

 from the anterior edge of the mouth to immediately behind the 

 velum. Hatschek conjectured that this canal opened into the 

 pharynx. [See MacBride, No. VIII. a.] 



This last observation has recently been confirmed by van Wijhe (No. 22) 

 who considers this organ, which he calls the oesophageal process, as the 

 remains of the original communication between the intestine and the left 

 anterior entoderm-diverticulum. 



We have already mentioned that, in the larval stages which follow 

 the stage with one primary gill-cleft, the number of primitive segments 

 is continually increasing through the formation of new segments from 

 the mesoderm-folds at the posterior end of the body. At the time 



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