78 SIDNEY F. HARMER. 



down into the mouth of the open calcareous funnel (fig. 4.7), 

 which is the condition of the young ovicell at the time when 

 it commences to expand distallj^, a tubular cavity can be seen, 

 which passes down to the basal lamina and represents the ovi- 

 cell in its zooecium stage. 



The upper wall of the expanded distal part of the ovicell is 

 thickly perforated by pores (fig. 1). These pass through the 

 calcareous layer; they are wider internally and constricted ex- 

 ternally, where they are closed by a cuticle, the pore being 

 entirely filled with a few cells. The pores are always much 

 more numerous in the upper wall of the ovicell than in the 

 walls of the zocecia, as Waters has pointed out (45, p. 277, 

 and in other pa|)ers). It is perhaps probable that the function 

 of these pores is mainly respiratory, to provide for the gas- 

 exchange which must be necessary for the development of the 

 great mass of larvae which are formed inside the ovicell. The 

 definitive assumption of the ovicell-character by a zooecium is 

 thus marked by a great increase of the porosity of its upper 

 wall. 



In some species, and notably in T. flabellaris (fig. 4, 

 ovicell 2) and T. a pert a, the more porous part can in some 

 cases be seen to be separated from the less porous part by a 

 sharp line. The proximal part has the appearance of a 

 zooecium ; and we thus have an external indication of the time 

 at which the fertile zooecium became definitely an ovicell. 



An ordinary zooecium was seen to become, sooner or later, 

 free from the common basal part of the colony, unless it 

 remained connected with it by forming one of a connate series 

 or of a fasciculus of zooecia. The corresponding process in the 

 ovicell is the formation of its tubular portion ending in the 

 orifice. This part, in all respects comparable with the up- 

 standing free part of an ordinary zooecium, is, however, 

 developed late, and can only be seen in ovicells which are 

 nearly mature. The ovicell, in fact, remains a part of the 

 common basal mass of the colony for a long period, during 

 which its distal end is expanding and increasing the space 

 available for the growth of the embryos. Before the distal 



