MUSEUM OF COMPAEATIVE ZOOLOGY. 1G9 



opening into a large anterior vessel. The pulsatile sac resembles a heart ; 

 the vessel opening from it, an artery which may distribute blood to the head 

 and cephalic tentacles. Of tlie true homology of these organs there is, how- 

 ever, some doubt. 



The body of the youngest Prionospio (Fig. 1) is composed of nine anterior 

 segments, bearing as many pairs of long provisional setae and four smaller 

 terminal segments without spines. Consequently, it will be seen that in the 

 youngest larva two segmented regions can be distinguished in the body ; 

 the anterior (ar) forming its great mass and bearing provisional setaj, and 

 the posterior (pr) relatively almost inconspicuous in size and vv-ithout spines. 

 The terminal segments of the latter are colored by bright red pigmentation. 

 The diameter of the intestinal tract narrows uniformly from the head to the 

 anal extremity, with little variation in different regions. 



Marked changes of most important character have taken place in the head 

 and l)ody of the next oldest larva (Figs. 2, 3, 4, 5). The arrangement of the 

 tentacles, seta3, and eye-spots on the head is about the same as iu the former 

 larva, and the dorsal walls have extended forward above the mouth into a lip 

 wliich had a rounded border, forming a structure which persists into the adult, 

 and will l;)e called, in subsequent larvaj of this worm, the prajoral lobe. This 

 nomenclature, however, does not imply that it is homologous with the struc- 

 ture which has the same name in certain other Annelid larvce. The body of 

 this larva has dropped, either normally or abnormally, most of its embryonic 

 setiP, and three regions, an anterior, a middle, and a posterior, have differentiated 

 themselves in it. Almost the whole of the body is still taken up as foimerly 

 by the anterior region. The middle region (mr) is smaller than the anterior, 

 has its walls more thickly pigmented, and retains the embryonic setaj even 

 when the larva is kept in confinement for some time. This region is formed 

 from tlie originally undivided posterior part of the former larva. The pos- 

 terior body region is the smallest of the three, and is the same as the non- 

 spinous part of the body of the youngest larva. 



The anterior region of the body in the present larva is composed of nine 

 segments, the lines of separation between each pair of which, liowever, are not 

 well marked. The lateral spines of this part are short and small. The body 

 walls are very transparent. On the sides of the body near the fourth pair of 

 spines there is a cluster of reddish pigment spots (ms), which persist even into 

 the oldest larva3 which have been taken (Fig. 13). 



The middle body region, which is developed from the original posterior por- 

 tion, is formed of four segments, the constrictions between which are deep 

 and well marked. The segments are sometimes swollen to a diameter 

 greater than that of the transparent anterior portion of the hoUj. The walls 

 are thicker than those of the transparent part described above, and are more; 

 densely pigmented with yellow and lirown. That portion of llie digestive 

 tract which lies in the middle b(jdy division is here considcreil the stomach. 

 The posterior division of the body has a smaller diameier than either of the 

 others, and is without appendages. It is, however, segmented, and later in its 



