LATER LARVAL DEVELOPMENT OP AMPHIOXUS. 229 



interesting to note that there are two kinds of possible homo- 

 logies, which may be called — 



1. Individual homology^ the one more commonly met with. 

 This is a morphological factor. 



2. Group homology, a physiological factor, but entirely 

 distinct from homoplas3^ 



It also follows that the peribranchial cavity of Ascidians is 

 in no sense homologous with the atrium of Amphioxus ; in fact, 

 this is a pure instance of homoplasy. 



Bateson (9) points out the probable homology of the proboscis 

 cavity and proboscis pore of Balanoglossus with the anterior 

 " prsechordal " vesicle and prseoral pit respectively of Amphi- 

 oxus; fjnd also he urges the homology between the gill-slits in 

 both cases. These identifications are likely enough to be 

 accurate, but I need not recapitulate the reasons for regarding 

 Amphioxus as more closely related to the Ascidians than to 

 Balanoglossus, though, since it forms a considerable feature in 

 the present paper, the absence of a true endostyle in Balano- 

 glossus might be pointed out. The forward position of the rudi- 

 ment of a notochord in Balanoglossus has probably no relation 

 whatever with the forward extension of the notochord in 

 Amphioxus. 



As to the affinities of Amphioxus with Ammocoetes, I will 

 only refer to the homology existing between the endostyle of 

 Amphioxus and the thyroid gland of Ammocoetes, which was 

 advocated first by W.Miiller and then by Dohrn(7),who worked 

 out the development of the latter structure. It is well known 

 that Dohrn regards the thyroid gland, and therefore the endo- 

 style, as representing a pair of gill-slits. This is of course a side 

 issue, and cannot be fully gone into here ; but it may be pointed 

 out that while the thyroid gland gives no reliable indication 

 of a double origin, the endostyle of Amphioxus is composed 

 of two distinct halves, which are at first upper and lower re- 

 spectively, but eventually become right and left — morphologi- 

 cally of course their relation to one another is always right 

 and left. 



As for the gill-slits in the two cases, attention may be drawn 



