1873.] 325 |Morse. 
rocks, it is almost impossible to determine whether they are dorsal 
or ventral. 
No one, however, would mistake these regions in the Lamelli- 
branchiate, or Gasteropod. 
In the Brachiopoda, with the exception of Lingula, there is a 
a great concentration of the body, quite unlike anything seen in the 
Vermes. Lancaster, however, describes, in the Annals and Maga- 
zine of Natural History, a worm, Chetogaster vermicularis, one 
of its chief points of interest being the exceedingly small number 
of segments, four or five only. 
Many Rotifera are also highly concentrated, or cephalized, with 
dorsal and ventral flattening, and with a chitinized integument. 
As to this concentration of parts in the Brachiopoda, it would be 
strange indeed if the worms alone should not show this concentra- 
tion of structure in some of their forms. This same diversity occurs 
in all the other groups, as in the Crustaceans, the highly cephalized 
Brachyurans, and the elongated Macrourans, and among the Cirri- 
peds, the concentrated and flattened Coronula, and the long pedun- 
culated Anatifa. Or among the Echinoderms, the flattened Mellita 
or Scutella, and the worm-like Holothurian. Or, again, in the 
Lamellibranchiates, the concentrated Isoecardia and the attenuated 
worm-like Teredo. Other examples might be given in the Polyps, 
Ctenophorez, Gasteropods, Insects, Fishes and Reptiles. 
Concentration, or cephalization of a structure, while modifying 
the character and functions of parts, and even obscuring their 
ready interpretation, can in no wise affect the relation of the animal 
sustaining such features, though it may account for certain peculiari- 
ties attending such conditions, in the same way that parasitism 
may account for the absence of certain organs, characteristic of 
related forms. 
Integumentary Organs. 
In Mollusks the tesumentary envelope is almost always extended 
to form a sac, or mantle, which is open below, sometimes resting like 
a cap upon the back, or better, extending itself in a wide membrane 
about the viscera, or it may hang upon the two sides of the body, 
split below in a median line, but not on the sides. 
This envelope, or mantle, usually secretes a shell composed of car- 
bonate of lime, and is attached to it by special muscles in limited 
areas, so that when these are separated, the envelope is found to 
