MICROSCOPICAL STUDIES. 99 



Explanation of Plate IX. 



Life-History of Obelia geniculates 



Fig. 1. Natural appearance of the fronds of the Hydroid-stage, as 

 they grow upon the surface of Laminaria. Life size. 



Fig. 2. A single frond of the same much enlarged, pr, perisarc ; 

 c, ccenosarc ; h, hydrotheca ; p, polypite ; b, bud devel- 

 oping to form a polypite later on (note that the perisarc 

 covers the entire surface of the bud ; as growth goes on 

 a thinning takes place at the apex, and finally gives way 

 to permit of the extrusion of the polypite's tentacles) ; 

 g, gonotheca; m, medusid buds in different stages, within 

 the gonotheca. 



Fig. 3. View from below of a freed medusa. 



Fig. 4. Diagram of same, 



Fig. 5. Diagrammatic vertical section through same. The section 

 follows the course of a radial canal on the right side, 

 while on the left, an interradial portion of the disc is 

 sectioned. 



Fig. 6. A Medusa, considerably older than that shown at Fig. 3, 

 seen reverted and from the side. It had just captured 

 an Arrow- worm (Sagitta), which is doubled up, the bend 

 being within the digestive cavity of the Medusa. 



Lettering the same for Figs. 3 — 6, viz : — a, atrium ; c.c, circum- 

 ferential canal ; en, endoderrn ; g.g, genital glands ; m, manubrium ; 

 o, mouth ; of, otocyst ; r.e, radial canal ; s.u, sub-umbrella ; t, ten- 

 tacle ; u, umbrella ; v. velum. 



Study XII. — On Polynoe propinqua as typical of the 



Higher Annelids. 



Polynoe projnnqua (Malmgren) is an elegant marine annelid, 

 short and slender in its proportions, rapid 'and lively in its movements. 

 When the tide recedes, its haunts are to be found Avherever boulders 

 litter the shore, and many a weary backache have I had through a 

 course of such stone-turning. It is easily recognized by reason of 

 two rows of oval overlapping brownish scales that lie upon its back, 

 and from under which project, oar-like, on either side, serried banks 

 of lovely translucent paddling-bristles. A pretty creature, but loath 

 to be captured. Alarmed at the approach of the forceps, it squirms 

 and makes desperate effort to escape, and indeed if it be lifted 

 roughly and by force, instead of being taken by some gentle snare, 

 lo ! it breaks into pieces and is to you but a worthless capture. 



