MICROSCOPICAL STUDIES. 41 



The prolongation of the ccenosarc into the gonangium is termed 

 the blastostyle. This fills but a small space and is little more than 

 a narrow column in the centre. In its walls are developed sperm or 

 ova as the case may be, and thus while it (the blastostyle) represents 

 really the medusiform person, the medusa itself is entirely abortive, 

 the reproductive organs alone being retained. 



Fertilization takes place as in Coryne, and then a peculiar 

 occurrence takes place. The apex of the blastostyle gradually 

 expands into a hollow globe with gelatinous walls, which pushes its 

 way through the aperture of the gonangium to hang from the 

 mouth as a miniature bladder. Into this pouch, the acrocyst or 

 marsupium — the fertilized ova are passed and therein undergo 

 segmentation. They pass out as planulas — which after a short free 

 life settle down, and by the usual process of growth and budding 

 complete the life-cycle by developing new colonial organisms. 



Sertularia obviously belongs to the Calyptoblastic or Thecate 

 Hydroids, the polypites being lodged in thecse, and it is significant 

 to note that it occupies the same relative position to Obelia in regard 

 to mode of reproduction, as Coryne occupies to Syncoryne among 

 Gymnoblastic Hydroids. In both Sertularia and Coryne there is 

 suppression of a medusiform stage. In Obelia and Syncoryne, free 

 sexual medusas are produced — a striking parallel. 



From the geological standpoint, Sertularia is of considerable 

 interest, as it seems to offer the most probable relationship to the 

 curious Graptolites, those most abundant fossils of Silurian rocks ; 

 the short sessile hydrothecge of Sertularia, giving its stems a serrate 

 appearance, offering great suggestive resemblance to the denticulated 

 margins of the fossils in question. 



Study XX. — The Cirripedia. 



Than the Barnacles or Cirripedes (" cirrus-footed "), few marine 

 animals are more familiar to dwellers by the sea ; the sessile forms 

 exist everywhere upon the littoral ; the stalked are known world-wide 

 as Ship-Barnacles, adhering to the bottoms of ships or to wave-tossed 

 timbers. 



The greatest diversity of form is found in this order, from the 

 great stalked forms and the mollusc-like Rock-Barnacles, encased in 

 shelly walls, that cover in multitudes all high-tide rocks ; from 

 parasitic species, more or less degraded, yet endowed with optional 

 freedom, down to curious bag-shaped parasites whose lives are bound 

 up absolutely with that of the host, and through whose vitals their 

 ramifying roots bend and twist. All agree, however, in larval history, 



