NEMATOCYSTS OF MICROSTOMA. 267 



as Grosvenor (1903) had claimed for the nematocysts of aeolids. 

 Martin (1908) must be given the credit for having first ap- 

 proached this question of the origin of the nematocysts of 

 Microstoma in the proper way, i. e., experimentally. 



Martin first made the observation that Microstoma do ingest 

 Hydras. "If a fasting Microstoma is placed in a watch-glass 

 which contains some small Hydra, it is almost certain in a short 

 time to come in contact with one of them. If the Microstoma 

 comes suddenly against the tentacles of the Hydra it contracts 

 itself immediately, and in this condition it may frequently be 

 killed by the discharged nematocysts. As a rule, however, the 

 Microstoma fixes itself for a short time by its posterior end in the 

 neighborhood of the Hydra, and everts its pharynx to its full 

 extent. 



'The Microstoma then swims over the surface of the Hydra, 

 usually attacking the lower part of its body with its pharynx 

 fully everted (vide Fig. 10). The Hydra then usually becomes 

 strongly contracted, and sweeps its tentacles over to the side on 

 which it has been attacked, though under these conditions the 

 tentacles do not grasp the Microstoma, but remain extended 

 almost parallel with its body, and it would appear as though the 

 pharyngeal secretion had a paralyzing action on the Hydra. 

 In many cases, after a time, the Microstoma leaves its prey, and 

 in such a case the Hydra does not seem much the worse for the 

 attack, but if the Hydra is of small size, it may be engulfed and 

 swallowed whole" (Martin, 1908, pp. 267-8). 



After this observation Martin's chief evidence is based upon 

 the histology of Microstoma. He finds that when both Hydra 

 and Cordylophora are fed to Microstoma there are found within 

 the tissues of the flatworm nematocysts peculiar to both kinds of 

 coelenterata that had been fed. Thus in sections of the Rhabdoc<zle 

 he discovers nematocysts in the endoderm, mesoderm, and ecto- 

 derm. Martin's methods of procedure had convinced me that 

 he had established that the nematocysts of Microstoma were 

 derived from Hydra, until I had considered the details of his work. 

 I hesitate, for example, to follow Martin when he says that 

 von Graff (1882) was wrong in saying that each nematocyst lies 

 in a single cell and that "Hallez had correctly described the 



