Mr. H. D. S. Goodsir on the Development of Acephalocysts. 483 



modes of propagation — one for the extension of the original group, 

 the other for the establishment of other groups. 



M The Echinodermata are represented among the Entozoa in a cu- 

 rious and interesting manner by the suctorial forms of that class ; that 

 is, by those forms of Entozoa which are endowed with these organs 

 as a means of adhesion or progression, such as Distoma, Tristoma, &c. 

 The lowest form in this suctorial tribe is the Diplozoon paradoxum 

 of Nordmann. I am inclined to consider Diplozoon as inferior to 

 Distoma and other suctorial forms, not from its analogies, but from 

 this circumstance, among others, that its whole organization is 

 double, and consequently less centralized. The Asteriadce, among 

 the Echinodermata, are represented in the Entozoa by Diplozoon and 

 other similar forms, which undoubtedly exist. The Tristomte are 

 represented by the flat Echinida, as the Scutellte. In both the Tri- 

 stoma and its echinodermatous analogue, the Scutella, we find the 

 disc imperfect in certain parts of its edge, indicating the remains of 

 a more divided or asteroid condition of the body. The Distoma are 

 the analogues of the true Echinidce. A starfish folded up upon itself, 

 so that the tips of its rays meet at one central point, constitutes that 

 form of the Echinodermata known as the Echinus. In like manner, 

 among the Entozoa Diplozoon holds the same relation to Distoma. 

 The former has two intestinal tubes and two mouths, one for each 

 body ; the latter has two intestinal tubes, and only one mouth. In 

 like manner also, the reproductive organs are similar. It thus ap- 

 pears that the Distoma is only a Diplozoon folded on itself, as Echi- 

 nus is an Asterias folded back. There are certainly some few points 

 of material difference between these two animals, a circumstance we 

 naturally look for ; but these, if properly observed, must be traced 

 to the difference of centralization. Distoma is therefore superior to 

 Diplozoon, as Echinus is to Asterias, having a more centralized 

 organization . 



" The Acanthocephalous Entozoa of Rudolphi are the analogues of 

 the Crustacea. The Echinorhynchi are typical of this group among 

 the Entozoa. On comparing an Echinorhynchus with a Crustacean, 

 such as a Lernean, the relation between them is so like that of affinity, 

 that they were at one time grouped together in the same class. 

 When the Lernean Crustaceans have passed their period of locomo- 

 tive existence and have become permanently fixed, their habits are 

 exactly similar to those of the Echinorhynchi, the only difference 

 being, that the former adheres to the external, and the latter to the 

 internal surface of the body of the infested animal. The Echino- 

 rhynchi have a number of short extremities or limbs near their head, 

 analogous to similar organs, or to the atrophied limbs of the Lernece. 

 There is this difference, however, between these organs in the two 

 sets of animals, namely, that in the one they have never become 

 developed at any period of life so as to suit the purposes of locomo- 

 tion, whereas in the other, and during its early stage of existence, 

 they were fully developed and employed as organs of prehension and 

 progression, but have only become atrophied during the stationary 

 or parasitic period of life. 



2K2 



