390 E. W. MACBRIDE. 
the various theories as to the relationships of Balanoglossus 
which have been put forward, together with an exposition of 
Professor Spengel’s own views on that subject. 
Confining ourselves in the first instance to the purely 
anatomical portion of the treatise, the main interest which 
attaches to Professor Spengel’s work is the proof he has given 
as to the extent to which homologous organs may vary within 
a small sharply circumscribed group; in other words, it is 
especially valuable from a systematic point of view. There 
are, however, several points of general interest which he has 
brought out which must be first dealt with. These concern 
the vascular, muscular, and skeletal systems. 
The vascular system is, according to Professor Spengel, 
nothing else than a system of communicating spaces and clefts 
devoid of any proper wall, the remnants of the embryonic seg- 
mentation cavity. Their muscular coat, when they have one, is 
derived entirely from the neighbouring organs. Thus the trunk 
ceelom supplies longitudinal and circular muscles to the dorsal 
and ventral vessels. The main part of the vascular system is 
in the form of a very close dermal plexus, and of a similar one 
between the splanchnic muscles and gut epithelium. These 
plexuses are connected with each other through the dorsal and 
ventral trunks by vessels running in the dorsal and ventral 
mesenteries. In the trunk the ventral vessel takes up the whole 
extent of the mesentery, and is in open connection with the 
enteric plexus. The ventral vessel is joined to the dorsal just 
behind the collar by a specially wide ring-shaped sinus. The 
musculature of the heart is supplied by the ventral wall of the 
vesicle, which Bateson called the sac of the proboscis gland. 
Spengel has, however, shown that it is an entirely closed vesicle, 
having no connection with the proboscis gland whatever. Itis 
the same as the so-called “heart,” or pulsating vesicle of the 
Tornaria. Spengel calls it ““ Herzblase’’—a term which I have 
ventured to translate “ pericardium.” The true heart! is a space 
between this vesicle and the notochord ; it communicates by a 
1 Spengel calls this space the central blood-space, and refuses the term heart 
to it on the ground that it is a mere split, and that its muscles are derived 
