26 
Tank No. 20. 
delight and the despair of the naturalist. For the wonderful form 
and beauty of their body is associated with such delicacy, that it breaks 
to pieces at the slightest touch. The fact that, nevertheless, especially in 
calm weather, specimens of Physophora (Fig. 28), Forskalia (Fig. 29), 
Hippopodms (Fig. 30) and others may be seen in the Aquarium, is 
due to the particular method employed in their capture (see foot-note). 
The Siphonophora are looked upon by most naturalists as free- 
swimming colonies. It is not rare to find individuals of one species thus 
united ; the corals form one of the most striking examples. The case of 
the Siphonophora, however, is somewhat different; here the individuals are 
not all similar and performing similar functions, so that each can live 
independently of the other; but the colonies are made up of very dif¬ 
ferently shaped (polymorph) individuals, each form undertaking one of 
the different functions, which have to be performed. Special nutritive 
polypes undertake the nutrition of the colony; special bell-shaped indivi¬ 
duals , like medusae, perform the swimming movements; true medusae 
are charged with the reproduction. In short, we have a "division of 
labour” taking place as it does among the ants and bees; but with 
the difference, that among them the polymorph individuals (workers, 
drones, queen) are separate one from the other, whereas in the Siphono¬ 
phora they are inseparably united. While Physophora (Fig. 28) gives the 
impression of medusae bound together by a slender thread or stalk, in 
the beautiful blue Sallee Man, Velella (Fig. 31), this common stalk is 
replaced by a wide horny disk overshadowing all the polypes and bear¬ 
ing on its upper surface a triangular crest, which catches the wind like 
a sail and enables the colony to drift along on the surface of the water. 
Physophora appears clearly to be a group of animals, Velella only one, 
but there are all transitions in the degree of union. 
CTEN0PH0EA. 
(Tank No. 20.) 
The Ctenophora agree with the Siphonophora and Medusae in the 
transparency of their body, a character which is of frequent occurrence 
in marine animals. There are some transparent Mollusks, there are 
transparent Annelids, transparent Crustacea and even transparent Fishes. 
The reason for this occurrence of transparent forms must be sought for 
in the advantage which they gain by such a character. Probably the 
advantage consists in the difficulty which their enemies have in seeing 
them, and in the facility with which they can surprise and capture their 
prey. These transparent forms*), even the delicate Ctenophora and 
*) These transparent forms live chiefly in the open sea and are therefore 
often grouped together under the name of Pelagic Animals. In a calm 
sea, and if the light be not too intense, they are found at the surface of the 
water; other conditions cause them to sink to some depth. They can thus 
not be caught regularly, and in stormy weather they may be missing in the 
Aquarium for some time. Usually the currents at sea bring them together in 
large numbers, and it is only necessary to steer into these ' f Correnti” to catch 
them in swarms. The large forms are carefully scooped up with buckets, and 
