350 Proceedings of the Royal Physical Society. 



covered by a thick layer of colletoderrn, which is con- 

 tinued over the body of the polyp, and which, when the 

 polyp retires within its tube, fills up the top of the 

 tube by its cushiony folds, so that the polyp is completely 

 hidden, and the funnel appears as it were closed by a valve. 

 The colletoderrn in my specimen was coated and impreg- 

 nated with mud. Mr Alder's specimen was covered with 

 grains of fine sand. I was at first inclined to believe that 

 this zoophyte was merely a variety of Atractylis repens, 

 which, with its medusoids, I have already described to the 

 Society ; but after it had been in captivity a few days, I 

 found that it was beginning to put forth ovisacs, one on 

 opposite sides of the polyp-stems (Plate XV. fig. 7). 



The mode of reproduction in this zoophyte is unique 

 amongst the Tubulariadae, though I have noticed and de- 

 scribed it in the Sertularias and Campanularias. 



The female generative sac of Atractylis arenosa resembles 

 that of Hydractinia ; it is a simple sac formed of ectoderm, 

 or the outer layer of the ccenosarc, enclosing a similar sac 

 of endoderm, the " placenta," the whole being covered by a 

 layer of scleroderm and colletoderrn. Between the placenta 

 and the ectoderm a large number of ova are developed, each 

 showing a germinal vesicle and spot (fig. 8). When the ova 

 are sufficiently advanced for extrusion frorn the generative 

 cavity, the investments of the sac are ruptured, the sac 

 assumes a long, cylindrical form (fig. 9), and a most labo- 

 rious process of parturition commences. With each pain 

 the ectoderm of the sac contracts laterally, like the bell of 

 a Medusa, and at the same time the placenta (fig. 9 c) is 

 dilated by fluid pumped into it from the somatic cavity of 

 the zoophyte, so that the ova, which are floating in a milky 

 fluid, are forced against the summit of the generative sac. 

 Meanwhile, another process has been going on, — the ex- 

 ternal surface of the summit of the sac has been secreting a 

 thick cap of gelatinous colletoderrn (fig. 9 d), which is to 

 form a nidus for the further development of the ova. The 

 contractions become still more violent, until the ova are 

 confined in a mass at the dilated upper part of the sac ; this 

 last is ruptured, and they are forced into the gelatinous cap, 



