2 A BIOLOGY OF CRUSTACEA 



The trilobites were at one time thought to be Crustacea, but they 

 are now regarded as being off the main line of crustacean descent. 

 They have, for instance only a single pair of antennae in front of 

 the mouth. The other head appendages do not differ very much 

 from those on the rest of the body, so that they cannot be said to 

 be separately modified for feeding, and there are no mandibles. 

 We can thus exclude the trilobites from the Crustacea, and for 

 the moment leave open the question as to whether they are any- 

 where near to the ancestral forms from which the Crustacea arose. 

 Before we tackle this problem it will be as well to have a look at 

 some of the early undoubted Crustacea. 



The best description of an early fossil crustacean is undoubtedly 

 Scourfield's description of Lepidocaris rhyniensis. This description 

 was built up from an examination of thousands of fragments of 

 Rhynie Chert. This is a rock which was produced about 300 million 

 years ago in a region with springs containing water saturated with 

 silica. Various animals and plants in this area have been incredibly 

 well preserved as the water evaporated and the Chert was formed. 

 The Chert is somewhat transparent, so that flakes can be examined 

 under the microscope and fragments of Lepidocaris can be seen, 

 often in fine detail, down to the hairs on certain bristles. Lepidocaris 

 was a small crustacean, not quite reaching a length of three milli- 

 metres. The body was narrow and tapered and was divided into 

 19 segments. The head bore two pairs of appendages in front of 

 the mouth; the antennules were small, but the antennae were large 

 and branched, looking very like the antennae of modern water fleas 

 (p. 5), so that they were probably used for swimming. The mouth- 

 parts consisted of a pair of mandibles which had milled surfaces 

 for breaking up food, and two pairs of maxillae, the second of 

 which were very small. In the male the first maxillae formed 

 claspers for gripping the female. Eleven pairs of legs lay behind 



itrniec 



Fig. 1. Lepidocaris rhyniensis, lateral view of female. Actual 

 length about three millimetres. (After Scourfield, 1926). 



