150 ZOOLOGY FOR MEDICAL STUDENTS chap. 



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

 over the surface of stones and shells on the seashore. Another is the 

 tiny Spirorbis, whose tube, coiled into a flat spiral, is so common on 

 seaweeds and rocks. In this case the stopper is hollowed out to form 

 a brood-cavity in which the eggs undergo part of their development. 

 The genus Sabella and its allies resemble the Serpulids in their general 

 character. One of their interesting features is their intense sensitiveness 

 to light impressions — a shadow falling on the expanded worm causing 

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

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

 cells become clumped together to form definite and complicated eyes 

 which show as round black dots on the gill. The genus Sabella itself 

 lives embedded in mud, its vertical tube of mud grains projecting upwards 

 beyond the general surface. Large worms of this genus a foot or so in 

 length maj' be frequently found a little below low-water mark, e.g. in 

 meadows of sea-grass in quiet bays and sea lochs. 



Development of Polygordius 



As an example of the mode of development of the marine annelids 

 we 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 

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

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

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

 by a process of invagination similar to that of Aurelia except that 

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

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

 becomes elongated, taking on an elliptical shape, and then it narrows 

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

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

 elongated opening is now represented by two distinct openings some 

 distance apart. Of these openings one persists as the mouth while 

 the other, though it closes temporarily for a short time, is represented 

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

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

 there is reason to suspect from hints given us by the study of the develop- 

 ment of various other animals that this represents the way in which the 



