No. 430.] THE COMMON SQUID, LOLIGO PEALII. 791 
flat elongate cells that encircle the vessel. This endothelium 
gradually merges into the typical endothelium lining the 
smaller arteries. Bergh! found a similar arrangement in the 
arteries of the pulmonate gasteropods, except that he ascer- 
tained definitely that the larger arteries and the heart do not 
possess an endothelium but are lined by muscle fibers. He con- 
cludes that the endothelial cells are undifferentiated (wngeformte) 
muscle cells. . There is nothing in the squid which tends to 
support such an explanation, for the larger vessels are almost 
certainly lined either by connective tissue or endothelium. 
The veins have the same structure as the arteries except 
that their muscular coat is thinner or wanting and that all 
have a typical endothelium (Fig. 3). 
They may readily be distinguished 
from the arteries by their thinner 
walls. In many places an artery is 
accompanied by a pair of veins which 
anastomose frequently. Fig. 4 repre- ric, 3.— Endothelium of a vein. 
sents a portion of such an artery with par 
its companion veins; the narrow initial caliber of the branches 
of the artery, due to the contraction of the intrinsic muscles of 
the vessels, should be noted. 
The nephridial sinus passes through the nephridial cavity 
and is invested by the secretory nephridial epithelium. Both 
surfaces of the sinus wall are increased by interdigitating 
evaginations from the lumen of the vessel and from the 
nephridial cavity. The consequent irregularity of the surface 
of the walls causes many artifacts in silver preparations, so 
that the cell outlines cannot be followed certainly ; neverthe- 
less, sections indicate that an endothelium is present. The 
nephridial epithelium of the sinus is columnar and is supported 
by a membrane, the proper wall of the sinus, which carries the 
nutrient arteries of the wall. 
The Capillaries. — The Berlin-blue injections which were 
intended to demonstrate the extent of the capillary system 
were perfectly successful. The injection fluid passed from 
the arteries through the tissues until it filled and flowed from 
1 Bergh, R. S. Anat. Hefte, I. Abth., Bd. x. 

