ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



ridge p- into a spike over the opening, so frequently seen winding 



:rface of stones and shells on the seashore. Another is the 



^nrorbis, whose tube, coiled into a flat spiral, is so common on 



,1 rocks. In this case the stopper is hollowed out to form 



itv in which the eggs undergo part of their development. 



'la and its allies resemble the Serpulids in their general 



( )nc of their interesting features is their intense sensitiveness 



urns a shadow falling on the expanded worm causing 



. draw back into its tube. This sensitiveness is due to sensory 



cells in tin- epidermis of the gills and in some of the allied genera such 



,-ells hem me clumped together to form definite and complicated eyes 



which >ho\v as round black dots on the gill. The genus Sabella itself 



nbedded in mud. its vertical tube of mud grains projecting upwards 



;al surface. Large worms of this genus a foot or so in 



may be frequently found a little below low-water mark, e.g. in 



grass in quiet bays and sea lochs. 



D I. \ ELOPMENT OF POLYGORDIUS 



in example of the mode of development of the marine annelids 

 \\e will take the case of the little marine worm Polygordius a member 

 of a small group of worms which on account of their very simple and 

 primitive character are usually separated off from the typical Polychaetes 

 as a group by themselves the Archiannelida. 



At sexual maturity the body of the Polygordius breaks up and sets 



netes, male or female as the case may be. Syngamy takes 



in the sea-water and the zygote undergoes the usual process of 



in giving rise to a blastula. The gastrula stage is reached 



process of invagination similar to that of Amelia except that 



a relatively much smaller portion of the blastula-wall becomes in- 



i to form the archenteron. The opening of this, the protostoma, 



beconv ted, taking on an elliptical shape, and then it narrows 



in the middle, its outline becoming that of a dumb-bell. Finally the 



Milr lips of the opening come together and completely fuse so that the 



M(.\\ represented by two distinct openings some 



irt. 01 these openings one persists as the mouth while 



li it closes temporarily for a short time, is represented 



by the amis. Consequently these two openings in Polygordius are to 



be regarded simply as the end portions of the original protostoma, and 



rora hints given us by the study of the develop- 



"ilier animals that this represents the way in which the 



