REPORT ON THE TETRACTINELLIDA. 
393 
It will be observed in the case of the Hexactinellida and Monaxonida that there is 
less difference in the percentage of stations for the two areas than in that of species. 
The influence of the East Indian region on the results is best shown by determining 
the richness of the line of dredgings from Stations 181 to 220, which lie within it. 
The results are given in tabular form for comparison : — 
Number of Species obtained from 
Thirty-six Stations (181-220). 
Per cent, of Species. 
Hexactinellida, 
34 
94-4 
Tetractinellida, 
22 
61-1 
Monaxonida, .... 
40 
IIM 
This comparative richness of archipelagos in species is a very striking fact, and 
deserves further investigation ; probably it is the result of several factors, amongst which 
we may suggest the relatively large extent of coast line involving a relatively wide 
margin of littoral vegetation, and consequently abundant food-supply, multiplication of 
individuals, and strenuous competition. Next the currents, tidal, oceanic, and surface 
drifts have an influence ; the struggle of the adults with waves and currents is an 
important factor in bringing about modification,^ and must make itself particularly felt 
in island groups where currents are numerous and powerful; currents may also exercise 
a distinct influence in bringing about the invasion of one locality by swarms of free- 
swimming larvae set free from another. A third factor may be found in the great variety 
in the nature of the sea-floor, and particularly the numerous changes in depth, pro- 
ducing great variety of climate, both as regards temperature, pressure, and intensity of 
light. 
We have, however, further to bear in mind that the richness of any rich area wiU 
appear unduly exaggerated in this investigation, since the Challenger dredgings were 
not made with that total want of intelligence which would be requisite to render them 
amenable to statistical treatment ; on the contrary, whenever a rich locality was discovered 
more than usually numerous dredgings were made, and were it not that these in the case 
of the Tetractinellida brought to light fresh individuals rather than species, the foregoing 
tables would be next to useless. 
^ A. R. Hunt, On the Influence of Wave-currents on the Fauna inhabiting Shallow Seas, Journ. Linn. Soc. Lond. 
(Zool.), vol. xviii. p. 262, 1884. 
(ZOOL. CHALL. EXP. — PART LXIII. — 1888.) 
Rrr 50 
