52 A. C. OUDEMANS. 
coat. It is, as well as the proboscidian sheath, always sur- 
rounded by the two well-known muscular coats, sending, as it 
were, two branches between the proboscidian sheath and the 
brain masses; it next gradually embraces the posterior lobes 
(as in fig. 48). 
The mouth then becomes visible and presses the circular 
muscular layer a little upwards. Transverse vertical muscular 
fibres appear between this layer and the proboscidian sheath, 
so that the lacuna is divided into two. These two lacune are 
again divided into four by a horizontal band of muscular fibres 
between the proboscidian sheath and the stomodeum. 
The two lacune beneath the horizontal band will again 
embrace the stomodzum, leaving here and there little lacune. 
The two lacune above the band, after the disappearance of the 
posterior lobes, acquire a circular layer of muscular fibres and 
diminish in size. The lacunz round the stomodzum here and 
there disappear, as if they terminated blindly ; some of them 
are still to be recognised, but the only distinctly visible lacune 
are the above-mentioned two with a circular layer and next to 
the proboscidian sheath: caetera desunt. (Two heads have 
served for this description; there was no difference between 
them.) 
11. Cerebratulus roseus (delle Ch.), Hubr. 
The precerebral region so strikingly resembles that of 
Cer. urticans, that I at first believed there was an error in 
the determination. As soon, however, as the circular muscular 
layer with its contents passes through the cerebral ring, differ- 
ences are to be seen. 
The two lacunze on the sides of the proboscidian sheath 
unite beneath the sheath, but instead of communicating di- 
rectly with the medial vessel, a part of the now single lacuna 
is cut off by the circular muscular fibres of the proboscidian 
sheath, so that a flat lacuna lies between this wall. This 
lacuna rises between the fibres, finally comes within the cir- 
cular coat, and is now the median vessel. So that the median 
vessel here, as in Cerebratulus marginatus, goes further 
