SIPUNCULOIDEA. 139 
short or very retracted; they form a small lophophore overhanging the mouth dorsally. In 
section each is roughly triangular, and besides the nerve each is provided with an extension 
of the blood sinus which lies round the brain. A similar sinus occupies much of the 
ventral lip, whose margins are, I judge, not broken up into tentacles. These sinuses aid in 
extending and keeping tense the tentacles and lip, when the introvert is everted. 
In the hollow of the lophophore lies a short channel, the “cerebral organ” of Ward’. 
This is shown in Plate VII. fig. 11. It is short and leads to the two eyes. 
The brain is compact, slightly hollowed ventrally by the blood sinus which on this 
surface closely ensheaths it. In the main the ganglion-cells form a dorsal cap. It gives 
off ventrally the usual cireumoesophageal commissures and anteriorly the conspicuous nerves 
to the tentacles and lower lip. 
The eyes are conspicuous, lying on the anterior surface of the brain, where that organ 
still retains its primitive connection with the ectoderm. Each consists of a cup-shaped 
pigmented layer of cells, densely black in colour; the little space surrounding the convex 
side of these cups, seen in Plate VII. fig. 11, is continuous with the cavity of the 
cerebral organ mentioned above. 
The systematic position of this Sipunculoid is a matter of some interest. Its structure 
resembles that of the genera Aspidosiphon and Cloeosiphon much more closely than it does 
that of any other genus, and it resembles Aspidosiphon much more nearly than it does 
Cloeosiphon. It however differs from <Aspidosiphon in the two most important features which 
characterize that genus. There are no anterior or posterior shields at all, but the anterior 
shield may perhaps be represented by the pad which bears and secretes the stony cap; 
if this be so, it may be noticed that this pad is more directly in a plane transverse to 
the long axis of the body than is that of any Aspidosiphon I have ever seen. The animal 
may be an abnormal form of Aspidosiphon but I doubt it. If it represent a true genus, 
this genus is one of the most easily recognized of all the Sipunculoids, even when the 
introvert is retracted. In many cases it is not possible at first sight to be sure whether 
one is dealing with a Sipunculus, a Phascolosoma, a Phascolion or a Physcosoma; but no 
one could make a mistake about this new form. For these reasons I have ventured to 
establish a new genus although I fully recognize the temerity. of such a proceeding when 
there is only one specimen to deal with. The beginning of the generic name I suggest 
indicates its stony cap, the end its nearest allies amongst the Sipunculoids. 
LITHACROSIPHON nov. gen. 
A Sipunculoid with a solid, conical, stony anterior end to its body. The stony cap 
borne on a pad, and showing traces of a laminated structure. No posterior shield. 
LITHACROSIPHON MALDIVENSE ouvov. sp. 
Longitudinal and circular muscles in bundles, the former of 26—28 strands which 
anastomose freely, Two retractors arising near posterior end. Spindle muscle present. Very 
numerous rows of slightly chitinized hooks. The hooks in each row two-pointed and very 
1 Bull. Mus. Harvard, xxt. 1891, p. 143. 
