THYMUS-LIKE STRUCTURES IN AMMOCOETES 133 



eratc type. However, a comparative study of the branchial 

 region of the lamprey larva with the same region in ascidians and 

 amphioxus on the one hand, and with the elasmobranchs on the 

 other, suggests that the branchial region of the lamprey larva 

 represents a transitional stage between the amphioxus and the 

 elasmobranch types. 



A few comparisons between the pharyngeal region of the 

 ammocoete and lower and higher forms may be found in the 

 literature. Dohrn ('84, '85) discussed the homology of the thy- 

 reoid of ammocoetes with the endostyle of ascidians and the 

 hypobranchial ridge in amphioxus and the circumoral ciliated 

 ring in the ammocoetes mth that of the ascidians. Cunning- 

 ham ('87) verifies the homologies Dohrn pointed out. Shipley 

 ('87) calls attention to the homology of the dorsal ciliated ridge 

 in ammocoetes and the dorsal lamellae of ascidians and the 

 epipharyngeal groove of amphioxus. 



The following considerations are based on my studies and 

 include besides the homologies just quoted a comparison of the 

 gills in these primitive animals: 



The large branchial cavity with, its medial gill arches of the 

 lamprey larva (text fig. 1) is very suggestive of the conditions 

 in the ascidians and amphioxus. In the ascidians there is a 

 central pharynx surrounded by a peribranchial cavity. The 

 two cavities communicate by means of numerous small pores, 

 the stigmata. It is an unsettled problem whether the peri- 

 branchial cavity is derived from ectoderm or endoderm. In 

 amphioxus there is a central pharynx which is partially sur- 

 rounded by an atrium (peribranchial ca\'ity). In this form the 

 two cavities communicate by means of definite gill slits. The 

 atrium of amphioxus is developed from ectoderm. In both forms 

 there is a ventral endostyle and a structure homologous with the 

 epipharyngeal ridge. The branchial cavity of the ammocoetes 

 corresponds to a fusion of the two separate cavities in the asci- 

 dians and amphioxus. The primitive characters of these sepa- 

 rate cavities, however, are still present. The central portion, 

 that is the part bounded laterally by the gill arches (fig. 1, 

 a.p.), corresponds to the pharynx of the ascidians and amphioxus. 



