DEVELOPMENT OF AGARICIA. 489 



disks, to move over the substratum with the aboral end downwards. 

 The failure of such larvae to become attached may have been due to 

 the surface of the glass vessel being unsuitable for fixation. As, 

 however, some of the larvae became fixed on glass while others did 

 not attach themselves even to rough surfaces, such as stones placed 

 in the vessel, the nature of the substratum does not seem to have been 

 the only factor involved. The experiment was tried of keeping larvae 

 in the dark and also under additional pressure, — eighteen inches of 

 water, — but in both cases they failed to attach themselves. 



When the larva is about to fix itself it becomes flattened and its 

 aboral end is applied to the substratum. If a stream of water from 

 a pipette be forced against such a larva, the animal may be made to 

 elongate its body again, provided it has not begun the formation of a 

 skeleton. Such larvae may remain in the current of the pipette, 

 appearing as if attached to the surface of the glass by an elastic strand. 

 This suggests that an adhesive mucous substance may be secreted 

 at the aboral end when it becomes applied to the glass. In the free- 

 swimming piriform larvae, especially when they are swimming close 

 to the substratum and apparently in a condition to affix themselves, 

 a concavity may be seen at the aboral end, giving that end the appear- 

 ance of a suction disk. 



Sometimes larvae flattened themselves out into disks at the surface 

 of the water. Such larvae never attached themselves to the bottom 

 or sides of the glass jar and almost all the individuals of such lots 

 became flattened out under the surface of the water. Such larvae 

 tended to fuse into " aggregations " (Duerden : 04) and also to go to 

 the sides of the vessel (surface tension). Many of these larvae lived 

 to secrete a skeleton with six well developed primary septa while still 

 floating at the surface of the water. 



2. Anatomy. 



a. Material, Methods and General Features. 



A considerable number of larvae, fixed in various mixtures, were 

 embedded in paraffin and sectioned either transversely or parallel 

 to the oral-aboral axis. Transverse sections were found to be by far 

 the most suitable for the study of the general structure. Nine of the 

 larvae so sectioned have been selected for detailed description in order 

 that the reader may have in as concrete a form as possible the data on 



