990 ON AN INSTANCE OP COMMENSALISM. 



recouverte cle bryozoaires " in the case of Petrochirus granuli- 

 mamis. The " bryozoaires " have probably been Con. commen- 

 sale, but they were not examined. 



In the volume dealing with the results of the inquiries of the 

 ' Travailleur ' and ' Talisman ' immense numbers of Polyzoa are 

 mentioned, but no reference to the present case. 



Summary and Conclusion. — The present Polyzoon from the 

 Cape Blanco region is a new species, viz. Gonopeuin coinmensale 

 Kirkpatrick & Metzelaar. It is most nearly related to G. lacroixii 

 Audouin . 



The paired " triangular spaces," paired knobs, tubercles, or 

 spines so commonly present in many species of Cheilostome 

 Polyzoa are probably aborted zocecial buds, more or less sup- 

 pressed or modified owing to lack of space for free growth. 



The association with the Crustacean Petrochirus gramdimanus 

 is a definite, not an accidental one, being a case of commensalism 

 in the sense of Yan Beneden. 



EXPLANATION OF THE PLATES. 



(The figures to be examined through a hand-lens.) 



Plate I. 



Fig. 1. Gasteropod shell inhabited by FetrocJiirus granuUmamis Miers, completely 

 encrusted bj' the Polyzoon Conopeum commensale Kirkpatrick & Metze- 

 laar from Cape Blanco. Nat. size. 



2. A shell and Polyzoan crust cut in half. Nat. size. 



3. Turritella shells inhabited by Hermit Crab, and each encrusted bj' young 



colonj' of Conopeum commensale : from Senegal. Nat. size. 



4. Conopeum. commensale. X 20. 



5 & 6. The same from another specimen, showing at the first separate triangular 

 tubercles and the single rectangular blocks. X 20. 



7. An older layer of zooecia broken into and viewed from the inferior or dorsal 



aspect, showing calcareous infillings of the opesias. X 20. 



8. Conopeum lacroixii, showing the two separate triangular " spaces " 



(boxes) at the base of each zocccium. X 20. 



9. Conopeum commensale. Transparent vertical section, showing laj'ers. X 3|-. 



(C/. fig. 2.) 



10. Conopeum lacroixii Audouin, showing separate triangular hollows : speci- 



men from Dovercourt, England. X 20. 



11. Memhranipora Uiherculata Bosc, encrusting two sides of alga from Algoa 



Bay ; showing remarkable blocks at the margin of colony, also ordinary 

 paired tubercles. X 20. 



12. Memhranipora tehuelcha d'Orbigny on Macrocystis pyrifera from South 



America ; showing tbe two spine-like tubercles to each zocccium. 



13. Thairopora armata MacGillivray, showing zocecia with paired boss-like 



tubercles and an avicularian cell with triangular hollow boxes above 

 mandible. X 20. 



N.B. — There is an avicularium with sharply-defined triangular 

 mandible about 8 mm. above the middle of the lower border of the 

 picture. The mandible and the triangular boxes above it have a frog's- 

 face-like aspect. In the ordinary zocecia the pairs of white boss-like 

 tubercles replace the triangular " hollows " or boxes. A lens is necessary. 



Plate II. 

 Fig. 14. Bulbous specimen secondarily overgrown with hydroid, etc. 



