Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . e more or less blendedtogether to form the head {Jig* 129, c), seven more obviously enterinto the formation of the thorax {g, ^), and theremaining seven constitute the abdomen or tail(a hY The Crustacea, with seven thoracic and sevenabdominal segments, form the sub-class Malacos-traca\\ but a few large species and a very greatproportion of the smallest members of the classhave the thorax and the abdomen composed re-spectively of a greater or a less number of consti-tuen
Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . e more or less blendedtogether to form the head {Jig* 129, c), seven more obviously enterinto the formation of the thorax {g, ^), and theremaining seven constitute the abdomen or tail(a hY The Crustacea, with seven thoracic and sevenabdominal segments, form the sub-class Malacos-traca\\ but a few large species and a very greatproportion of the smallest members of the classhave the thorax and the abdomen composed re-spectively of a greater or a less number of consti-tuent segments than seven : these Crustacea formthe sub-class Entomostraca. The best-markedgroup, that including the largest species of this sub-class, is named Xiphosura, because the last seg-ment of the body forms a long three-edged sharp-pointed weapon : itis typified by the Limulus, or Molucca crab, in which the head andthorax are more completely blended together than in the true crabs,which they resemble in the general form of the body ; but they arepeculiarly distinguished from all other Crustacea by having the office. Cyraothoa. * CCXXXII. f In the larger species of Crustacea, where the chitine is combined with sucha proportion of carbonate and phosphate of lime as to be firm and brittle, it is,as Aristotle has observed, less hard and less brittle than the shell of the mollusks,whence that philosopher called those Testacea Ostracoderma, but gave to theCrustacea the name of Malacusti aca, which name is still retained for that divisionof the class which alone was known to the Greek naturahst. CRUSTACEA, 299 of jaws performed by the basilar joint of the six pairs of thoracic legs,which surround the mouth, the internal branch of those legs formingan ambulatory and prehensile member. The large cephalo-thoracicsegment is protected above and laterally by an expanded crescenticshield, obscurely divided by two longitudinal impressions into threelobes, supporting the organs of vision
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