Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals. Cells in primitive animals (sponges in particular) shows similarities to collared choanoflagellates, as well as pseudopod- producing amoeboid cells. Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear nearly 700 million years ago, during [...]
Figure 12.4 shows the structure of a lobster, a representative crustacean. A crayfish, which is a freshwater form, differs morphologically from the lobster only in minor respects. The lobster has 20 segments, the first 13 of which are united on the dorsal side in a combined head and thorax, or cephalothorax. A heavy shield, or carapace, arises from the head [...]
In this article we will discuss about the structure of spider with the help of a suitable diagram. Spiders, like most arachnids, live on a completely liquid diet. All are predatory. The prey is bitten and often paralyzed by the chelicerae, and then enzymes from the mid-gut are poured out over the torn tissues to produce a partially digested broth. [...]