THE EGG. 31 



The first of them appears to be placed at the ventral end of 

 myotome 10, and the last at the ventral end of myotome 35 just in 

 front of the atriopore. They are contained in coelomic sacs, 

 which in development are derived from the ventral ends of the 

 myotomes, and to the wall of which they are attached. 



The phylum possesses only a single genus* Amphioxus Yarrell (Branch- 

 iostoma Costa). It is found in all seas. About ten species are known. 

 They differ in the number of myotomes, the presence or absence of a 

 caudal expansion of the median fin, the presence of gonadial sacs on one 

 or on both sides of the body, the continuity of the right metapleur with 

 the anal fin or the cessation of both right and left metapleur behind the 

 atrial pore ; the presence or absence of fin-rays and fin-ray spaces in the anal 

 fin. There appears to be a considerable range of meristic variation in some 

 of the species (Punnett). A. lanceolatus Pallas, Europe and most seas ; 

 A. bassanum Giinther, right metapleur continuous with anal fin, gonads 

 on right side only, anal fin with fin-rays and fin-ray spaces, Bass Straits ; 

 A. cultellum Peters, like the last, anal fin with chambers but without rays, 

 Torres Straits ; A. lucayanum Andrews, like the last, but without caudal 

 fin, hind end of body being a urostyle-like process without myotomes, and 

 fin-rays and chambers absent from anal fin, Bahamas. 



It has been suggested that one or two species are pelagic, but this is 

 uncertain. 



Development. The development of Amphioxus presents some 

 remarkable features, and contrary to our usual custom we have 

 decided to give a full account of it in this work. Though 

 strangely similar in many of its features to the type of develop- 

 ment found in the Vertebrata, it presents some very marked 

 features of difference. Of these we may at once mention the 

 small size of the ovum, the archenteric origin of the coelom, the 

 absence of any nephridial apparatus comparable to that of 

 the Vertebrata, the origin of the gonads from the myocoel, and 

 the extraordinary asymmetry of the larva. 



Our account is based upon the important researches of 

 Hatschek and Willy, who, as is well known, worked at the 

 species found in the Pantano at Faro in Sicily. 



The egg is small ('1 mm. in diameter), is surrounded by a vitel- 

 1'ne membrane and contains but a small quantity of yolk, which 

 is uniformly distributed. Only one polar body is attached to the 

 ovum after deposition ; it is probable that this is the second, 



* By some zoologists the phylum has been broken up into genera and 

 rkaldy, Q.J.M.S., 37, 1895, p. 303), but this, considering 

 er of species and points of difference, seems hardly necessary 



