MARINE INFUSORIA. ' 11 



Birmingliam through Mr. Tho3. Bolton's excellent microBcopic agency, 

 is a form admirably adapted for the observation of this same funda- 

 mental type of structure, the carapace valves in this instance being 

 moreover composed of elegantly reticulated polygonal facets that amply 

 repay microscopic iuveatigation. 



4. — Zootlmmnium alternans, C. and L., (Plate IV., Fig. 5.) — This very 

 beautiful tj'pe, identical with the ZootJtammium plumosum of Dr. Strethill 

 Wright, the Z. spirale of P. H. Gosse, (" Tenby," Plate IV., Fig. b.) and, 

 possibly, the Zoothamnium niveum of C. G. Ehi-enberg, (in which case 

 tliis last-named specific title must take precedence of the others,) was 

 obtained in considerable abundance attached to SertularUe and other 

 Hydroid Zoophytes brought up with the dredge, from various depths. 

 Some of these colonies were remarkable for their size and luxuriance of 

 growth, those obtained from deeper water, fifty fathoms, being most 

 notable in this respect. Compared wth the more famihar fresh-water 

 h]^eciea,Zoothamniumarbuscula, (sometimes found in sea- water,) Z.aZtt'ruaTW 

 may be immediately distinguished by the mode in which the secondary 

 branches az-e disposed with relation to the main or axial stem of the 

 compound colony. In the present instance, these secondary branches 

 are given off alternately, sometimes from opposite sides, and sometimes 

 in a spiral manner, producing, in this latter instance, an exceedingly 

 elegant, tall, plumose colonial stock, or " Zoodendrium," which may be 

 indefinitely prolonged. Not unfrequently, and more especially in 

 examples obtained near the shore line, the height of the entire colony 

 does not much exceed the breadth, and, the branches originating at 

 regular intervals on opposite sides of the main stem, the zoodendrimn, 

 as a whole, presents a flabeUiform or espaher-like contour. In Zootham- 

 nium arbuscula, on the other hand, aU the secondary branches diverge 

 radially or close to one another from the apex or distal teraiination of the 

 main, or axial stem, which latter, being once produced, does not 

 increase in length. 



5. — Zoothamnium dichotomum, St. W., (Plate IV., Fig. 6.) — One or two 

 colonies of a type that, so far as it is possible to predicate, appears to be 

 identical with the form very briefly described in " Pritchard's Infusoria" 

 under the above title, were obtained from the lower depth of fifty 

 fathoms. The zoodendrium, in this instance, is built up by the more 

 or less regular dichotomous division of a somewhat slender primaiy, or 

 axial stem, the ultimate divisions, or pedicles, which support the 

 associated zooids being of considerable length. In no other species of 

 Zoothamnium, as yet described, does the supporting stem exhibit such a 

 weU-marked dichotomous plan of growth. The Z. Cienkowskii, of 

 WrzesniowsM, and Z. elegans of De Udekim, pz'esent the nearest 

 approach to the same, but in both of these there exists a considerable 

 distinction in the proportionate length and thickness of the axial stem 

 and supporting pedicles. In every instance, two or three larger repro- 

 ductive zooids (Plate IV., Fig. 6a) were found attached towards the bases 

 of the main branches, which were conspicuously distinct from the 



