764 SUMMARY OP CURRENT RESEARCHES RELATING TO 



only that the head of the worm corresponds to the hinder end of the 

 Echinoderm-larva, but that the mode of formation of the endoderm in 

 the lower Metazoa has no phylogenetic significance. The so-called 

 Accela will have to be regarded as degenerate forms, though they are 

 neither parasitic nor sessile. All these difficulties may, however, be 

 overcome by the supposition that the Gastrcea does not represent the 

 most primitive form of Metazoa, but a later stage which succeeded 

 upon that of Metazoa without a digestive lumen, and with an internal 

 digestive parenchyma. From this point of view the parenchymatous 

 larvae of sponges and hydroids, as well as the lowest acoelous Turbellaria, 

 must be regarded as being closely allied to one another, and as repre- 

 senting the oldest Metazoa. It was not till later that there were 

 developed from them animals with a differentiated enteric canal, like 

 the hydroid polyps of the present day, where we see repeated the 

 most important phylogenetic phases (migration of endodermal cells, 

 formation of a solid parenchyma, and later development of an enteric 

 lumen). The earlier development, in the course of time, of the 

 endoderm may be compared to the early formation of such organs as 

 the vertebrate notochord, and the change in time may well be allowed 

 to have exercised a not unimportant influence in the process of 

 gastrulation, and the large blastopores of some forms may be best 

 ascribed to adaptive modification ; to this also would appear to be 

 due the appearance of several gastrula stages in the development of 

 one and the same animal form. 



In conclusion it is pointed out that, while in some cases we 

 have this hypergastrulation, in others there are formed pseudo- 

 gastrulae, that is, stages similar, but not exactly corresponding to 

 gastrulae. Here may, perhaps, be ranged E. van Beneden's rabbit- 

 gastrula, or his Dicyema, while an interesting example is to be found 

 in the Cyclostomatous Polyzoa, the gastrulas of Barrois being in fact 

 preceded by a much earlier stage in which the endoderm is really 

 formed. The author's notes on Discoporella radiata bring to an end a 

 most suggestive essay. 



Symbiosis of Dissimilar Organisms.* — G. Klebs here discusses 

 symbiosis with mutual adaptation, which is generally represented by 

 forms which are very widely separated, and often belong the one to 

 the animal and the other to the vegetable kingdom. Instances are 

 cited both among plants and animals; among the latter the corals are 

 perhaps as remarkable as any, and here we frequently find that while 

 the guest is dependent on the coral, the latter does not seem to re- 

 quire the guest ; when, however, it is present, it may lead to very 

 considerable modifications in the form of its partner. The specific 

 characters of Heteropsammia michelini are to be referred directly to 

 the presence of an Aspidosiphon. 



Another set of relations is well shown by the case of the crab 

 Pagurus prideauxii and the Actinian Adamsia palliata ; when the 

 former changes its shell, it seizes on the anemone by its chehe and 

 carries it off to its new home. The latter is completely adapted to 



* Biol. Centralbl., ii. (1882) pp. 385-99. 



