160 A REPORT ON THE SIPUNCULOIDEA, COLLECTED BY DR WILLEY. 



DESCRIPTION OF PLATE XVIII. 



Figure 1. Aspidosiphon khmzingeri, Sel. & Biilow, x 2. The introvert is partially everted. 



Figure 2. Aspidosiphon ravus, Sluiter, x 4. The introvert is completely extended and shows 

 the spines near the base and the rows of hooks near the head. 



Figure 3. One of the spines of A, rams from the base of the introvert, highly magnified. 



Figure 4. A view of a piece of the introvert of Sipunculus australis, Kef., showing the 

 so-called " hooks," and their relation to the rows of glandular papillae. Much magnified. 



Figure 5. A transverse section through three of the cuticular structures called "hooks" of 

 S. australis showing the cuticular caps overlaying the special elevations of the skin. 



Figure 6. Sipunculus billitonensis, Sluiter, x 1. 



Figure 7. A much magnified view of the anus of S. billitonensis. 



Figure 8. Sipuncidus cumanensis, Kef., x 1. This specimen shows the violent contractions of 

 the circular muscles which often give specimens of this species a characteristic appearance. 



