190 6- 1907.] Copepoda living as Messmates with Ascidians. 359 



which now seek the shelter and protection of the Ascidians 

 were formerly as free-living as the others. 



But though these Lichomolgi and a few other species have 

 retained their active habits along with a certain amount of 

 freedom within the branchial chamber of the larger Ascidians, 

 the majority of the Copepoda found in such situations are 

 usually inactive and sluggish in their movements, and have 

 their limbs more or less imperfectly developed. Dr Brady, 

 the eminent British authority on the Entomostraca, referring 

 to these Copepoda, observes that they are indeed, in all 

 probability, Cyclopidae which have become modified in build 

 by their inactive habits and the restricted boundaries of their 

 dwellings ; and it is evident that under these conditions the 

 long antennae and limbs of the free-swimming species would 

 be an encumbrance, or at any rate be of very small service, 

 and one would almost be inclined to believe that these 

 appendages must still be in course of degradation, owing to 

 constant disuse. 



The Copepoda chiefly referred to in the foregoing remarks 

 are those usually found within the comparatively roomy 

 branchial chamber, but there are other species which hitherto 

 have only been obtained in the alimentary tract, and are in 

 consequence more restricted in their movements, and they 

 also exhibit a rather more retrograde development. There 

 also appears to be another and a rather interesting difference 

 between a few of these forms whose environment is so limited 

 and others that are favoured with a more liberal allowance of 

 space. The females of the former species carry two external 

 ovisacs, which are slender and sometimes considerably longer 

 than the animal itself, as exemplified in Aplostoma ajfinis 

 mentioned in my recently published ' Catalogue of Forth 

 Crustacea.' On the other hand, the females of several of the 

 species living in the more roomy branchial chamber are not 

 furnished with external ovisacs, but have the fourth thoracic 

 segment, with which the fifth is usually coalesced, enlarged on 

 the dorsal aspect to form a pouch in which the ova are 

 enclosed. This pouch when packed with ova becomes greatly 

 distended, and forms a conspicuous part of the animal, owing 

 to the colour of the developing eggs showing through the thin 

 integument. It is because of this modification of the fourth 



