A HISTORY OF ESSEX 



of structure which assists them in breathing air. The colour of 

 P. ratzeburgii is described as grey, with a row of six white spots along 

 each side of the back. 



For the Amphipoda which, like the Isopoda, are fourteen-footed 

 sessile-eyed malacostracans, there are no available printed records, but 

 I can scarcely omit to mention the occurrence of Jassa pulchella, Leach, 

 Corophium crassicorne, Bruzelius and Caprella linearis (Linn.), since speci- 

 mens kindly given me by Dr. Sorby indicate that these well-known 

 species are as abundant in Essex waters as in many other localities. Also 

 on the oyster-testing excursion above mentioned, I was myself able to 

 obtain specimens of Hyperia galba (Montagu) from its accustomed 

 habitat in the common jelly-fish Aurelia aurita. 



The results thus brought together, however scanty in themselves, 

 are at least suggestive that Essex, as might be expected from the position 

 and character of its coast-line, will be found to provide very abundant 

 and attractive resources for students of marine carcinology. For those 

 interested in the crustacean fauna of inland waters, there is no need to 

 rely on conjecture or presumption. In regard to freshwater Entomostraca 

 the labours of Mr. D. J. Scourfield have placed this county in the front 

 rank. Twelve years ago there was not a record traceable for any single 

 species of this group with definite locality assured to it. At present, 

 although the subtle manoeuvres, the diminutive size, or the absolute 

 rarity of some species may have left them to be gleaned by future 

 researches, already Mr. Scourfield has been able to enumerate more 

 species from this county than have yet been recorded from any other, 

 having found in Essex more than a hundred species out of a total of 

 less than two hundred known from the British Isles at large. 



To make any account intelligible of this great number of species, 

 an outline. must be given of their classification. There are three principal 

 companies, called Branchiopoda, Ostrac6da, Copepoda. The Ostracoda, 

 or shelly group, have their unsegmented bodies boxed up in a pair of 

 valves, as if they were little molluscs. The Copepoda, by name and 

 nature oar-footed, have the body segmented and not enclosed in a 

 bivalved shell. These however, when parasitic, often become subject 

 to strange vagaries of structure, which set definition at defiance. The 

 Branchiopoda are so called from the branchial or respiratory character 

 proper to their limbs. They include three subdivisions, Phyll6poda, 

 Clad6cera, Branchiiira, among which difference of appearance is often 

 quite as prominent as likeness. The Phyllopoda are again divided into 

 sets far from closely resembling one another, since one set has a carapace 

 and another set has none, while the third has the body almost enclosed 

 in a pair of valves. Cbirocephalus diaphanus^ already noticed, belongs to 

 those that have no valves nor carapace, yet being of all our freshwater 

 Entomostraca though unadorned adorned the most. 



The Branchiura are a very small and rather perplexing group, 

 represented in our islands chiefly by the long-known Argulus foliac e us 

 (Linn.), a disk-like parasite on various fishes and tadpoles, with its 



210 



