272 DB J. H. ASHWORTH AND DR JAMES RITCHIE ON THE 



that the two tentacles are turned forward, while the sporosac is still invested by its 

 ectotheca, and that they then hold exactly the place of an umbrella (mesotheca) 

 between endotheca and ectotheca. They are, in fact, the radiating canals of the 

 medusa reduced to two and developed as free tubes, instead of being immersed in 

 the walls of an umbrella. . . . The free sporosac of Dicoryne is thus a medusa 

 reduced to the condition of a manubrium and two opposite radiating canals. In 

 order, indeed, to convert it into an ordinary medusa, little more is necessary than to 

 suppose the number of tentacles increased to four by the symmetrical development 

 of two others, their extremities connected by a circular canal, and their sides by a 

 continuous muscular membrane (umbrella) inflected at its fore margin so as to 

 form a velum." 



Allman was led to this view by the interpretation he put upon the parts of the 

 sporosac. He regards the external cuticular envelope as homologous with the 

 " ectotheca " of an ordinary medusiform gonophore, and the ectoderm layer in which 

 the oocytes or spermatogonia are differentiated is, in his view, the " endotheca." 

 Consequently the tentacles of D. conferta, lying as they do between the supposed 

 " ectotheca " and " endotheca," occupy exactly the position of the umbrella or 

 " mesotheca " and its canals, in a normal medusoid gonophore. 



But this view appears to us to be untenable and to have been supported by false 

 homologies. The typical medusoid gonophore exhibits, according to Allman's 

 terminology (i860, p. 3), an " ectotheca," a " mesotheca," and an " endotheca." " The 

 ectotheca is a simple extension of the ectoderm of that part of the zoophyte from 

 which the gonophore arises, and it encloses either a sporosac or a medusoid " 

 (loc. cit.). Of the other layers " the inner (endotheca) is the equivalent of the 

 ectoderm layer of the manubrium of the medusa ; the middle layer (mesotheca) 

 corresponds "to the umbrella, and like it may have a system of canals more or less 

 completely developed in it" (1871, pp. 39, 40). 



If these terms are to be employed in support of any homology, their application 

 must be uniform. The " ectotheca," for instance, must be understood in its original 

 significance as applied by Allman in the case of a medusiform gonophore, where it is 

 represented by the external investment — an ectodermal layer which arises, along 

 with the ectoderm of the ex-umbrella, from the ectoderm of the blastostyle. The 

 " ectotheca" may secrete on its outer surface a delicate cuticular film, as, for instance, 

 in Bougainvillea. This ectoderm is totally distinct in origin from that covering the 

 manubrium, for the latter arises from the thick mass of ectoderm cells — the 

 " Glockenkern " — at the distal end of the medusa, in which, by the development of 

 the future umbrella cavity, the manubrium becomes differentiated from the ectoderm 

 of the sub-umbrella. In a fully differentiated medusa (" phanerocodonic gonophore") 

 there are four layers of ectoderm from without inwards, namely, the " ectotheca," 

 the ectoderm of the ex-umbrella, that of the sub-umbrella, and that covering the 

 manubrium. A similar condition exists in a medusoid gonophore (" adelocodonic 



